Review on the Treatment of Cancer with the help of Control Drug Delivery System

Oral cancer refers to tumors developed in the lips, hard palate, upper and lower alveolar ridges, anterior two-thirds of the tongue, sublingual area, buccal mucosa, retromolar trigons, and floor of the Pharmaceutics 2019, 11, 302; doi:10.3390/pharmaceutics11070302 www.mdpi.com/journal/pharmaceutics Pharmaceutics 2019, 11, 302 2 of 29 mouth [1]. The majority (>90%) of oral cancer are carcinomas with squamous differentiation arising from the mucosal epithelium, thus called oral squamous cell carcinomas (OSCCs) [2,3]. In 2018, 354,864 new cases of cancer of the lip and oral cavity were diagnosed, and 177,384 individuals lost their lives due to such types of cancer across the globe [4]. As per the Canadian Cancer Society and the Canadian Dental Association, OSCC has increased in Canada in both genders since the mid-1990s; 4700 new oral cancer cases and 1250 oral cancer-related deaths were observed in Canada in the year 2017 alone [5,6]. Most frequently diagnosed at advanced stages (about 60% of patients have advanced stage disease at the time of initial diagnosis, OSCC remains one of the most challenging problems in head and neck oncology, and is still a disfiguring and fatal disease with bleak 50% to 60% five- year disease specific survival rate [7,8]. Because of its anatomic site, OSCC growth and treatment have important effects patient quality of life, encompassing impairment of the majority of the most critical functions (i.e., speech, swallowing, taste.), appearance and feeling of self; they correlate with significant functional morbidity despite cure of the cancer [3,9]. There have recently appeared new trends in the OSCC patient category including young patients (under 50 years old), specifically with human papillomavirus (HPV)-positive cancers [10,11]; a consistent alteration in the OSCC sex ratio with a concerning rise in OSCC incidence and mortality in women [12]; and the significance of new, previously unidentified factors, including the circadian clock disturbance in the OSCC onset and development [13–16]. OSCC has classically been linked to risk factors like tobacco and alcohol use; however, HPV, a renowned cause of cervical cancer, has recently developed as an etiological reason for a subgroup of head and neck squamous cell carcinoma (HNSCC), especially in individuals who do not have the conventional risk factors [17,18]. Most (60–80%) of HPV-related head and neck cancers are oropharyngeal squamous cell carcinomas (including the tonsils and the base of the tongue). Different types of HPV have been identified by recent research to be present in both benign and malignant oral cavity lesions [19–22]. HPV diagnosis is important in treatment planning for oropharyngeal cancer (OPC) patients [23–25]. Within OPC, clinical behaviors and patient outcomes differ significantly between HPV test positive and negative patients. In high/late-stage patients, HPV positivity has emerged as an important prognostic indicator that is crucial for determining treatment selection, with an HPV positive diagnosis leading to reduced toxicities and better outcomes [26]. Conversely, a notable subset of patients with early-stage OPC are HPV negative, their cancer quickly advances to advanced metastatic tumors and does not respond to standard of care with dismal outcomes and survival. Patients with long-term exposure of the entire upper digestive tract mucosa (cancerization field) to carcinogenic stimuli (e.g., from tobacco, alcohol, and betel quid chewing) are at increased risk for multiple primary tumours’. Oral squamous cell carcinoma is the second most frequent cancer in transplant recipients (e.g., treated for leukaemia, lymphoma, multiple myeloma, etc) [27]. The traditional methods of oral cancer treatment include surgery, which is the preferred treatment, ionizing radiation which is the most common non-surgical therapeutic method, or a combination of radio-, chemotherapy, and surgery [28]; surgical resection results in permanent disfigurement, changed sense of self and debilitating physiological effects, considerable functional impairment, and morbidity, whereas chemo- and radio-therapies are associated with marked toxicities, all of which impact patient health and quality of life. All these treatments are effective for treatment of the main tumor but applied with palliative purpose in advanced stages with metastatic disease, with major side and adverse effects [29]. Even with improvements in surgery, chemotherapy, and radiotherapy for the treatment of HNSCC, the prognosis for this disease has not been meaningfully enhanced in the past 50 years [8]. Therefore, the creation of new therapeutic strategies or alterations of established strategies is imperative to enhance patient individual survival and health results, while earlier tumor detection remains a challenge and priority. The oral, head, and neck cancer is an immunosuppressive disease (with a diminished absolute lymphocyte count and impaired antigen-presenting function) that disrupts the patient's Pharmaceutics 2019, 11, 302 3 of 29 natural immune response, blocking tumor cell recognition and immune-mediated removal [30]. Immunotherapy, a newly emerging cancer treatment modality, has demonstrated potential as a cancer treatment modality in addition to others, in patients who have failed multiple previous treatment modalities, as a result of the success of immune-modulating agents in refractory solid tumors [31,32]. The objective of immunotherapy as an anticancer treatment is either to stop the routes by which cancer cells evade the immune system or to augment the patient's immune responses against the tumor cells [33]. Anti-cancer immunotherapy involves: (1) systemic treatment, which is systemic activation of immunity including systemic cytokine administration, cancer vaccines, or adoptive cell therapy; (2) local- based treatment, which is founded upon local immune status change including modification of the immunosuppressive tumor microenvironment, either immune checkpoint or small molecular inhibitors [34]. Immune-modulating treatment options that are available to treat head and neck cancer focus on a multitude of immune processes and pivotal checkpoints, such as cytotoxic T-lymphocyte associated antigen-4 (CTLA4), and program death (PD-1) and its ligand (PD-L1); alternative strategies employing immune modulating molecules as well as combinatorial trials examining these agents in the first-line treatment and early-stage disease are in the process of being developed [35,36]. Since HNSCC tumors have been demonstrated to weakly present tumor antigen (TA) on the cell surface, monoclonal antibodies that improve TA presentation are one such area for targeted therapeutics [37]. Nivolumab and pembrolizumab, two anti-PD-1 drugs, recently approved in the second-line setting as monotherapy for platinum-refractory recurrent/metastatic HNSCC, have demonstrated efficacy in clinical trials [30,38]. Other targeted agents employing epidermal growth factor receptors (EGFR, overexpressed to a high level in 80–90% of HNSCC) inhibitors, including cetuximab, bevacizumab, and erlotinib, have indicated OSCC patient survival improvement [39]. Even though immunotherapies have been promising, new therapeutic strategies or refinements in clinical trial design that are personalized on the tumor/patient profile are greatly required to counteract the intrinsic and acquired resistance of the tumors and to solve/prevent their side and adverse effects [40]. Novel immunotherapeutic strategies can be hopeful in accomplishing long-term disease control in the response population, whereas the low efficacy and high toxicity in a few patients can be a serious problem [33,34,38]. In general, employing immunotherapy can be difficult because of auto-immune side effects, inconsistency in tumor responses rate, and economic cost [36]. The efficacy of immune agents utilizes nano-based drug delivery systems (DDSs) by direct targeting of the cancer cells, intracellular penetration, and enhancing the immunogenicity of antigens [41]. Up to now, there are few studies about the use of DDS in combination with immunotherapy for the treatment of HNSCC or OSCC. Hirabayashi et al. and Maeda et al. created anti-EGFR antibody-conjugated microbubbles for the treatment of HNSCC and OSCC, respectively [42,43]. These reports demonstrated encouraging outcomes for future uses of combined immunotherapy with DDSs. DDS have been designed as a new approach with the aim to maximize the advantages of the existing anticancer treatments, while reducing their unwanted toxic side effects on the healthy cells. For example, the chemotherapeutic drugs possess a number of drawbacks regarding oral bioavailability, stability in natural environments, and non-specific bio-distribution, which reduce their therapeutic efficacy [44,45]; them side effects can be important especially in elderly patients with crippling comorbidities. For example, parenteral delivery of chemotherapeutic agents enables the drug access through the bloodstream, hence to influence other non-cancerous organs/tissues in the body, in addition to the tumor; the degree and clinical impact of such non-specific effects are difficult to foresee. Side effects like nausea, vomiting, hair loss, infections, and diarrhea are usual in patients undergoing chemotherapy. Radiotherapy alone or combined with the chemotherapy may be used to treat the main tumor; reduce the size of the tumor before surgery (neoadjuvant treatment; see also: chemotherapy is also used in the neoadjuvant setting); as adjuvant therapy in an attempt to ensure the effectiveness of the primary reatment in order to prolong the survival time and lower the chance of recurrence; or to alleviate pain or manage symptoms of advanced oral cancer (palliative therapy). A patient's reaction to neoadjuvant therapy can decide which adjuvant therapy is chosen. Side effects of radiation therapy because of temporary Pharmaceutics 2019, 11, 302 4 of 29 or permanent injury to normal tissues are fatigue, sore or dry mouth and swallowing difficulty, dental issues (tooth decay), change in taste, loss of appetite, nausea and vomiting, nerve damage, pain, infection, osteoradionecrosis, trismus, lymphedema, and alopecia [29]. These may hamper the eating time and speech time and may cause additional complications like dehydration and malnutrition, social withdrawal, depression and anxiety, which affect the quality of life of the patient. Traditional therapeutic methods require an improvement in the bioavailability and targeted delivery to the tumor area (for a pre-determined duration) to counteract and evade the toxic side effects of the drugs [46]. Our team examined the conceivable anticancer effects of antacid drugs, including proton pump inhibitors and histamine 2 blockers frequently prescribed in HNSCC individuals to control acid reflux, a condition that is responsible for postoperative complication or radiotherapy complication. Our result in a big cohort study revealed that common clinical use of these two drug classes antacids in HNSCC patients was associated with improved survival; surprisingly our analysis revealed histamine 2 receptor antagonist class use as an independent prognostic factor for recurrence-free survival in oropharyngeal tumors HPV-positive patients [47]. Current research in our laboratory is examining the capabilities of these drugs to enhance the effectiveness of standard therapies, especially in advanced HNSCC [48,49]. A novel strategy to enhance the effectiveness of chemotherapeutic agents is the administration of medications in a time-dependent way (chrono-chemotherapy). Administration timing is proving to be as important as the dosing volume of chemotherapy [50]. The administration time (morning vs. evening) affects drug toxicity and therapeutic response because human body physiology is regulated by circadian clock rhythms [51]. Anticancer chemotherapeutic drugs docetaxel, doxorubicin, fluorouracil, and paclitaxel have been recently identified by the World Health Organization as drugs that act on circadian clock genes (Bcl2, Top2a, Tyms, and Bcl2 respectively). Therefore, they can be utilized in chrono- chemotherapy for the treatment of oral cancer [52]. One recent study demonstrated that chrono-chemotherapy of a regimen of Docetaxel, Cisplatin, and Fluorouracil (DCF) contributed towards minimizing the intensity of the side effects of each of these agents [53]; OSCC patients experienced less vomiting, nausea, and neutropenia with evening DCF administration compared to morning administration [53]. Therefore, it appears promising that chrono- chemotherapy can diminish the severity/degree of side effects of certain chemotherapeutic drugs, which can be utilized as a new therapeutic approach in oral, head and neck cancer patients and elsewhere. Another method which proved to be promising in evading the side effects of traditional anticancer drugs while increasing their therapeutic effectiveness is the targeted drug delivery system consisting of natural and/or synthetic polymers for delivery of chemotherapeutic agents to the tumor site. Targeted drug delivery systems have the potential to increase drug bioavailability and bio-distribution at the site of the primary tumor. DDS is capable of releasing a bioactive molecule at a specific site with a specific delivery rate. Targeted DDS for oral cancer could thus improve patient compliance, enhance drug efficacy while shortening treatment time, and thereby lower healthcare costs. In vivo research has demonstrated that targeted DDS may also enhance the half-time of otherwise degradable drugs like peptides and proteins, extending their local activity [54]. Our discussion of the most promising anticancer drug delivery strategies is organized in three sections as follows: first, the traditional anticancer drugs are discussed with respect to their oral administration and DDS formulation potential; second, an overview of popular carriers used in DDS for oral cancer treatment is given; and third, the potential of various drug delivery techniques for OSCC is considered.

1.1 History of Drug Delivery Technologies

Prior to 1950, all drugs were prepared in pill or capsule forms that released the loaded drug immediately on contact with water without any possibility of controlling the drug release kinetics. Smith Klein Beecham launched the first sustained release product in 1952 that could control the drug release kinetics and provide 12-hour efficacy[2]. The technology, called the Spansule technology, provided control of the drug release kinetics at a predetermined rate. In the early period when the new controlled drug delivery technology started, several new names were used to define newer formulations with slight differences from each other. Controlled release formulations consisted of those with sustained release, timed release, extended release, and so on. Of them, the term "sustained release" has been employed most extensively than any of the other names. These names, however, are employed interchangeably today. Following some decades of developments in drug delivery technologies, the slight differences in the functionalities that various names represent have rendered themselves redundant. The background history of controlled drug delivery discipline is outlined in Table 2. The majority of the drug release mechanisms fundamental knowledge, particularly oral and transdermal dosing forms, was achieved within the first generation (1G) of the development between the years 1950 and 1980. During this phase, four mechanisms of drug release were established, which speeded up development of many oral and transdermal controlled release products. The most common Yun et al.J Control Release. Author manuscript; available in PMC 2016 December 10. Author Manuscript Author Manuscript Author Manuscript Author Manuscript mechanisms were dissolution-controlled and diffusion-controlled systems. Osmosis-based formulations enjoyed a fleeting popularity, but the number of osmosis-based products is orders of magnitude less than the other two. The ion-exchange mechanism separates from the rest, but it was not helpful unless it is merged with diffusion-controlled mechanism. Even now, most oral once-a-day products are designed following the dissolution- or diffusion-controlled mechanism. Oral delivery being the most convenient drug administration mode, oral sustained release products will continue to thrive. Unlike 1G drug delivery products, the second generation (2G) technologies have been developed less effective, based on the quantity of clinical products formulated. One reason for this is that the 2G technologies address harder-to-formulate drugs. For instance, injectable depot formulations of biodegradable poly(lactic-co-glycolic acid) (PLGA) are intended to provide peptide and protein drugs for a month or more. Depot formulations are generally hard to manage the initial burst release, which tends to release 50% of the total drug in the initial day or two[3]. During 2G era, pulmonary delivery systems for insulin have been also formulated. Pulmonary insulin delivery system was formulated, but lower bioavailability demanded delivery of several times more drug than parenteral injection required. This, subsequently, led to unforeseen side effects that, together with other reasons, led to withdrawal of the product from the market[4]. In an alternative approach, numerous self-regulated insulin delivery systems were constructed over the years[5–8]. Self-regulated insulin delivery systems perform quite well in the laboratory environment, but they quickly lose the function after being implanted in vivo. The decade of the 2G period (i.e., 2000~2010) has been devoted to tumor-targeted drug delivery by nanoparticles. The apparently encouraging nanoparticle strategies based on small animal models failed in many clinical trials[9, 10]. The marginal successes of the 2G technologies must be studied meticulously in order to get the existing 3G technologies ready for potential clinical applications.

2. Anticancer Agents for Oral Cancer Treatment Formulated in Drug Delivery Systems

Although the majority of the oncological therapies are conventionally delivered intravenously, some anticancer agents have been introduced and licensed by the USA Federal Drug Administration (FDA) in recent years for oral delivery [55]. Pharmaceutics 2019, 11, 302 5 of 29 Delivery of chemotherapeutic drugs as pill or gel is an appealing method to improve patient compliance. This mode of delivery is also preferable when the therapy necessitates drug exposure for extended durations [46]. Oral delivery of most anticancer agents is unfortunately hindered by the physicochemical properties of the drug, mainly poor aqueous solubility [56,57]. Yet, the majority of the chemotherapeutic agents administered intravenously can be administered through other delivery routes when formulated in appropriate carrier (bio)materials [58]. Properly designed DDS can be employed to formulate chemotherapeutic agents for local (e.g., applied at the tumor site) or intravenous administration with increased effectiveness compared to the conventional intravenous dosage. The following is a description of the most prevalent anticancer agents employed for the treatment of oral cavity and oropharynx cancer patients [59], which have already been explored for their administration employing controlled and/or targeted DDS with satisfactory results.

2.1. Paclitaxel (PTX)

Paclitaxel (Taxol) is an antineoplastic drug that acts through cellular inhibition of growth. Oral administration of PTX is difficult due to its poor solubility and decreased permeability through the intestinal epithelium/mucosa that restrict its absorption. When PTX is given intravenously, which is the most frequent administration route in the clinic, its distribution within the body is extremely wide, leading to serious side effects like liver impairment [60]. To enhance its absorption, Lee et al. developed a platform using the chemical conjugation of PTX to the low molecular weight chitosan, which enhanced PTX's water solubility because of the availability of chitosan and its greater retention time in the gastrointestinal (GI) tract [61]. Tiwari and Amiji documented nano-emulsion formulations of PTX to enhance its oral bioavailability; the nano-emulsion delivery of PTX led to a substantial increase of the PTX content in systemic circulation compared to control (aqueous solution of PTX), indicative that such a formulation will improve the oral bioavailability of hydrophobic drugs like PTX [62]. Dong and Feng further incorporated montmorillonite in poly(lactic-co-glycolic acid) (PLGA) for synthesis of nanoparticles as a delivery tool for PTX; the montmorillonite-PLGA nanoparticles supported a higher cellular uptake and effectiveness of PTX compared to that of PLGA nanoparticles alone implying that the montmorillonite-PLGA nanoparticle formulation may prolong the residence time of PTX in the GI tract [63].

2.2. Cisplatin (DDP)

Cisplatin is a chemotherapeutic with an established advantage for the treatment of several types of human cancers such as oral, head and neck squamous cell carcinoma, bladder, lung, ovarian, breast, and testicular cancers. Cisplatin induces cancer cell apoptosis through its ability to crosslink with purine bases on DNA, disrupting DNA repair mechanism, and inducing DNA damage [64,65]. Since its administration has proven to be related to serious side effects like renal failure, attempts have been numerous to design this drug in an oral sustained release system [64]. Cheng et al. took advantage of the capacity of the low pH-responsive porous hollow nanoparticles of Fe3O4 to be as a drug delivery vehicle for site-specific delivery of cisplatin; their platform, based on encapsulation of the cisplatin into porous hollow Fe3O4 nanoparticles, not only prevented cisplatin deactivation by plasma proteins and other biomolecules en route to the target site, but also gave control of the rate of release of cisplatin through modification of the nanoparticle pore size and pH [66]. Yan and Gemeinhart synthesized encapsulated cisplatin poly(acrylic acid-co-methyl methacrylate) micro-particles for regulated release of cisplatin, and their system made cisplatin retain its activity over longer intervals of time [67]. A cisplatin derivative with comparable chemotherapeutic profile, Carboplatin, has also been researched singly or as a component of nanoparticle drug delivery systems in an attempt to reduce its unwanted side effects [68].

2.3. Doxorubicin

One of the most effective anticancer drugs utilized for the treatment of many cancer forms, due to its capacity to attack proliferating cells, both cancerous and non-cancerous. Pharmaceutics 2019, 11, 302 6 of 29 Its toxicity towards non-cancerous cells restricts its use since it can lead to cell death in significant organs like heart, brain, liver, and kidney [69–71]. Drug delivery strategies aimed to reduce DOX side effects while taking advantage of its anticancer activity with greater therapeutic efficacy. For example, Li et al.encapsulated DOX in dextran nanoparticles to target specifically tumor cells with the hope that these intelligent nanoparticles would enhance drug loading efficiency and deliver the drug at a specific site directly into the nucleus of the cancer cell [72]. She et al. employed dendronized heparin nanoparticles conjugated with DOX as a pH-sensitive drug delivery system for cancer therapy. These nanoparticles exhibited strong anti-tumor activity against a 4T1 breast tumor model without harming healthy organs [73]. Collectively, this evidence indicated that the encapsulation of DOX in nanoparticles had potential for decreasing toxicity towards normal cells and enhancing its antitumor efficacy.

2.4. Docetaxel

Docetaxel (DTX), a potent anticancer agent, is generally administered intravenously in cancer patients due to its extremely hydrophobic nature, but it exhibits poor oral bioavailability due to P-glycoprotein (P-gp)-mediated efflux and first pass effect. In order to overcome these limitations, Sohail et al. synthesized a chitosan scaffold wherein folic acid and thiol groups were grafted onto chitosan totarget cancer cells and enhance permeation through the gastrointestinal tract [74]. They also synthesized silver nanoclusters in situ, which enabled the formation of core-shell nano-capsules with the hydrophobic DTX as the core and the silver nanocluster embedded chitosan as the shell; this strategy led to a DTX carrier system appropriate for oral delivery of DTX to cancerous tissues [30].

2.5. Methotrexate

Methotrexate (MTX), an anticancer chemotherapy antimetabolite agent, is a folate antagonist that inhibits the formation of purines and pyrimidines, thus causing inhibition of proliferation of the malignant cells. MTX is applied in the treatment of a range of cancers, including oral, head, and neck cancer, acute lymphocytic leukemia, non-Hodgkin's lymphoma, choriocarcinoma, osteosarcoma, and breast cancer [75,76]. Oral administration of MTX results in a systemic bioavailability of about 35%, much lower than with parenteral administration [77]. MTX oral administration has serious side effects (diarrhea, ulcerative stomatitis, hemorrhagic enteritis, gastrointestinal perforation) resulting from the inhibition of cell proliferation. Kumar and Rao developed MTX in proteinoid microspheres to increase its targetability and bioavailability, with the hope that such microspheres would be capable of delivering MTX and other drug agents that are susceptible to degradation, in gastric condition [78]. Paliwal R et al. entrapped MTX in solid lipid nanoparticles (SLNs) made of stearic acid, glycerol monostearate, tristearin, and Compritol 888 ATO; the MTX loaded SLNs greatly enhanced the bioavailability of MTX by shielding MTX from degradation in the severe gastric conditions [79].

2.6. Fluoropyrimidine 5-Fluorouracil

Fluoropyrimidine 5-fluorouracil (5-FU), another FDA approved anticancer drug, inhibits vital biosynthesis processes or interferes with DNA or RNA, restricting their normal function. This drug has proved to be successful in the treatment of different forms of cancer, such as oral, head and neck cancer, colorectal, and breast cancer [80]. Li et al. created a biodegradable controlled release system made up of PLGA nanoparticles, which ensured a sustained continuous release of 5-FU. According to their findings, these nanoparticles were able to increase the oral bioavailability of 5-FU while reducing its local gastrointestinal side effects [81]. Minhas et al. synthesized a pH-responsive controlled release system for delivery of 5-FU, through the preparation of a chemically cross-linked polyvinyl alcohol-co-poly(methacrylic acid) hydrogel containing 5-FU, that allowed the release of 5-FU at pH 7.4, with the potential to be used as an oral drug delivery vehicle for 5-FU in cancer therapy, especially colorectal cancer [38].

3. Delivery Of Poorly Water-Soluble Drugs

Impotence of water to dissolve drugs was one of the most critical challenges in drug development, and it continues to be a reality even today. Discussion regarding drugs that poorly solubilize involves knowledge regarding the significance of drug solubility. The terminologies employed by U.S. Pharmacopeial and National Formulary to define rough drug solubilities in water are displayed in Table 4. It is routine terminology used to classify drugs that fall in the "practically insoluble" class. For such drugs aqueous solubility is 0.1 mg/mL or less, i.e., 100 µg/mL or less. Most candidate drugs are poorly soluble in water, and therefore, many of the candidate drugs are not converted into clinically effective formulations. Evaluation of 200 oral drug products revealed that practically insoluble drugs constitute nearly 40% of the total drugs [19]. Distribution of these drugs efficiently through the GI tract for therapeutically useful bioavailability is a critical concern. The dissolution rate of virtually insoluble drugs can be extremely slow such that dissolution exceeds the GI transit time leading to therapeutically ineffective bioavailability [20]. Technologies for dissolving poorly soluble drugs in water have been investigated over decades, and some of them are described in Table 5. Poorly soluble drugs possess inherently low water solubility, and accordingly, appropriate excipients are incorporated to enhance the solubility by employing surfactants, polymer micelles, hydrotropic agents, complexing agents (e.g., cyclodextrins and proteins), cosolvents, and lipid formulations (e.g., self-emulsifying systems)[21–23]. For weakly acidic or basic drugs, pH can be managed to enhance the drug solubility. Alternative to enhancing the drug solubility, drug dissolution kinetics may be improved by choosing proper polymorph, preparing solid dispersions (i.e., retaining amorphous form of the drug with polymers), decreasing drug particle size, and enhancing wetting with surfactants. Of these, the solid dispersion method has been most commonly applied for its ease of preparation and effectiveness [24–26]. Preparing drug nanocrystals has also been often employed, since the enhancement of bioavailability by increasing the surface area of the drug crystal led to enhanced bioavailability [23]. The surface area increases proportionally as reduction in the particle size of drugs. The drug solubility is a built-in characteristic and thus must not vary as the dissolution kinetics is enhanced. But enhancing the dissolution kinetics may lead to enhanced bioavailability of oral drugs. Increased dissolution of the drug can yield the dissolved product in adequate amount in time enough to replace those drugs absorbed from the GI tract, thus enhancing bioavailability. The issue of poor water solubility is even more critical for intravenous formulations. For instance, most anticancer drugs are very poorly water soluble, e.g., <1 µg/mL, and hence they are typically dissolved in organic solvents. Paclitaxel and docetaxel are typical examples of poorly soluble drugs making injectable formulations challenging. There are different injectable formulations of paclitaxel: Taxol using Cremophor® EL [27], Abraxane® from paclitaxel- albumin complex[23, 28, 29], and Genexol® with PEGPLA polymer micelle [30]. Taxotere, in delivering docetaxel, a paclitaxel derivative, is dissolved in polysorbate 80 suspected to induce hypersensitivity [31, 32]. Cremophor EL, one of the excipients employed to make paclitaxel soluble, has been known to produce severe hypersensitivity reactions and kill patients if the patient is not adequately preconditioned[27]. New drug delivery systems for insoluble drugs without employing organic Yun et al.J Control Release. Author manuscript; available in PMC 2016 December 10. Author Manuscript Author Manuscript Author Manuscript Author Manuscript solvent plays a significant role in advancing promising novel drug candidates into clinical uses and better utilization of known drugs.

3.1 Peptide/Protein/Nucleic Acid Delivery

Macromolecular drugs, e.g., peptides, proteins, and nucleic acids, are typically administered by parenteral administration. They are too large to pass through the intestinal epithelium, i.e., to be absorbed from the GI tract[33]. Various efforts have been made to shield them from the severe acidic environment of the stomach by enteric coating, and from enzymatic breakdown by incorporating enzyme inhibitors. These efforts, however, do not solve the actual problem that proteins must be broken down into small molecules by enzymatic degradation before they can be absorbed[34, 35]. Nanoparticles have been said to be able to be translocated through M-cells in Peyer's patches and enterocytes in the villus section of the intestine, but how much particle is absorbed has been debatable [36]. The amount absorbed is too small and too non-reproducible to be of any therapeutic value. These macromolecular drug primarily administered via parenteral routes. In recent years, novel strategies have been tried to administer them by non-invasive, or minimally-invasive routes, e.g., pulmonary, nasal, and transdermal delivery [37]. Macromolecular drugs typically possess very short half-lives ranging from minutes to hours, and sustained release for months is therefore necessary, which necessitates depot formulations. There are over a dozen depot formulations administered via parenteral routes. Zoladex® Depot (goserelin acetate), Lupron Depot® (leuprolide acetate), Sandostatin LAR® Depot (octreotide acetate), Nutropin Depot® (somatropin), Trelstar® (triptorelin pamoate), Suprefact® Depot (Buserelin acetate), Somatuline® Depot (lanreotide), Arestin® (minocycline HCl), Eliaard (leuprolide acetate), Risperdal® CONSTA® (risperidone), Vivitrol® (naltrexone), Ozurdex® (dexamethasone), and Bydureon® (exenatide). The fact that there are only a few depot formulations, compared with thousands of oral sustained release formulations, reflects the difficulty involved in the development of parenteral depot formulations. Most of these formulations have the initial burst release, leading to the first peak blood concentration much higher (up to 100-fold) than the therapeutically effective concentration at the steady state (i.e., following drug concentration at the steady state following the first peak). Therefore, it is strongly needed to enhance the technology of controlling the drug release profiles. The capability of controlling drug release kinetics becomes increasingly important as the drug loading. Depot formulations intended to have longer duration require higher drug loading. Therefore, patient-friendly depot formulations should have higher drug loading with controllable drug release kinetics for a long-period of time, upto 1 year, or even longer.

3.2 The Initial Burst Release from PLGA Depot Formulations

Examples of pharmacokinetic profiles of two clinically applied depot products are illustrated in . Every PK profile can be segregated into two parts: the initial burst release phase (red arrows in Figure 1) and therapeutically effective phase (green arrows in Figure 1). The Y axis of Figure 1 is in the log scale, and the peak concentration in the initial burst phase is approximately 100 times greater than the concentrations present in the therapeutically effective range. This observation raises several questions. Firstly, is there actually a necessity for 100 times greater drugYun et al. J Control Release. Author manuscript; available in PMC 2016 December 10. Author Manuscript Author Manuscript Author Manuscript Author Manuscript concentration in the first day or two than the well-established therapeutically effective drug concentration? Second, does the initial burst release have any contribution to the drug's efficacy at the steady state? There is no scientific basis to support that the initial burst release is required for therapeutic effect. The initial burst release is merely a result of emulsion techniques of microparticle manufacture at hand in the past couple of decades. Some would argue that the initial peak concentration in blood may be necessary for therapeutic efficacy. This however cannot be true, because it suggests that injecting the same drug daily without the peak concentration would not work. This, of course, is not true. It is the drug concentrations in the region of therapeutics that is critical. Controlling the initial burst release still isn't easy, but enhanced understanding on the emulsion methods and recent advances in new microfabrication techniques have enabled one to minimize or eliminate the initial burst release.

3.3 Targeted Drug Delivery Using Nanoparticles

Nanoparticle-based drug delivery systems have been employed widely over the past few decades. A search on SciFinder with "drug delivery nanoparticle" yielded 19,950 references between 1995–2014 (Figure 2). Of these, 57% are linked with the term "target" for targeted drug delivery or targeting. Evidently, most of the research on nanoparticle-based drug delivery has been targeted towards targeted drug delivery, primarily tumor-targeted drug delivery. The first interest in nanoparticulate drug delivery systems came from the capability of producing nanoparticles of diverse size and shape, and the capability to engineer the physicochemical and surface properties in order to prepare smart nanoparticles. Most of these systems have performed very well in the laboratory when cell culture systems were employed for drug delivery testing. The systems also performed fairly well in small animal models, primarily xenograft mouse models. The nanoparticle systems with encouraging outcomes in those models have not been carried forward to clinical studies[38, 39]. The existing nanoparticles cannot dictate their fate upon intravenous administration. The so-called "targeting" by nanoparticles is a misnomer, since the existing nanoparticles cannot navigate to a desired target, but are merely distributed throughout the body by the blood circulation[40]. Only a very minor portion of the total administered nanoparticles reach the target site, primarily by accident. The principle of the enhanced permeability and retention (EPR) effect is often quoted whenever nanoparticles are employed for tumor drug delivery. Most studies have not, however, quantitatively determined the actual quantity of drugs that reach the target tumor, and therefore, there is no quantitative data on the role of the EPR effect in targeted drug delivery. The mouse tumors are typically 1~2 mm in size which is as big as the liver, but only a minor part, in the range of roughly 1% of the administered total dose, of the so-called target nanoparticles reach the tumors, while most end up at the liver [41]. It is necessary for nanoparticle systems to be a clinically effective drug delivery tool, they could have to be configured differently from ones demonstrating potential in mouse experiments. The conclusions drawn from mice, with limited milliliters of blood, might not be applicable to human with 5 liters of blood. Moreover, the relative size of a tumor within a mouse would typically be greatly different from a tumor in human. This Yun et al. Page 7 J Control Release. Author manuscript; available in PMC 2016 December 10. Author Manuscript Author Manuscript Author Manuscript Author Manuscript gross scale differences must be taken into account when experimental animal models are utilized and their data interpreted. Nanoparticles can yield some surprises, despite the fact that the expected targeting has not been seen so far. The nanoparticles, with appropriate surface modification, can modulate the ziodistribution, thus potentially changing the toxicity profiles of the identical drug. Indeed, decreasing the drug's toxicity, or side effects, through nanoparticle engineering formulations might be an even more effective means of exploiting nanoparticles' special properties. Doxil®, the PEGylated liposome drug product, is such an example. It was licensed by the U.S. Food and Drug Administration not due to its enhanced drug efficacy, but due to its decreased cardiotoxicity [42]. Given the challenges of making the targeting capability seen in mouse models translatable to clinical applications, it might be possible to use nanoparticle formulations for minimizing the toxicity. This is possible not just by modifying the biodistribution, but also by enhancing the water-solubility without the need for toxic organic solvents. Good examples of this are Abraxane® and Genexol® mentioned above. Organic solvent-free formulations, e.g., Cremophor EL or polysorbate, are definitely more preferable, particularly when the resulting therapeutic effect is roughly equivalent [43].

3.4 Self-Regulated Drug Delivery

Self-regulated drug delivery, especially self-regulated insulin delivery, is still one of the most significant technologies to be developed. Suppose millions of diabetes patients can manage their glucose level for months with a single injection of self-regulated insulin delivery system, rather than multiple injections of insulin per day. There are a number of self-regulated insulin delivery systems that have been developed over the years and function well in the laboratory setting[5–8, 44, 45]. As soon as they are brought inside the body, however, their functionality decreases by hours. The glucose sensor, which plays a critical role in sensing the fluctuating glucose level, becomes lesseffective because of protein adsorption and cell adhesion, and the insulin delivery module becomes less effective after every cycle[18, 46, 47]. It has been quite decades since the idea of self-controlled insulin delivery began, but the progress has been slow. This is primarily because of the biological barriers which the body presents to theimplanted device[48]. If the biological barriers are not well understood and the new deliverysystems are not designed to overcome those, development of self-regulated insulin deliverysystem will be an idea for a period of time. The biological barriers to be overcome are the onesof retaining glucose sensor specificity and sensitivity within the biological environment. Anothermain requirement is to construct an actuator which releases a right quantity insulin quickly with automatic turn-off function[13].

3.5 Carriers for OSCC Drug Delivery Systems

Carrier-based drug delivery systems are used for controlled release of drugs while providing improved selectivity and effectiveness, and reduced side effects compared to the chemotherapeutic agents alone. Different carrier systems based on nanoparticles, nanolipids, and hydrogels are discussed here, each with unique advantages and disadvantages (Figure 1). Additionally, exosomes have been recently introduced as potential carriers of chemotherapeutic agents for oral cancer treatment. The benefits and drawbacks of each carrier system are summarized in Table 1. Pharmaceutics 2019, 11, 302 7 of 28 co-poly(methacrylic acid) hydrogel loaded with 5-FU, which enabled the release of 5-FU at pH 7.4, with the potential for being used as an oral drug delivery vehicle for 5-FU in cancer treatment, particularly colorectal cancer [38]. 3. Carriers for OSCC Drug Delivery Systems Carrier-based drug delivery systems are used for controlled release of drugs while providing improved selectivity and effectiveness, and reduced side effects compared to the chemotherapeutic agents alone. Different carrier systems based on nanoparticles, nanolipids, and hydrogels are discussed here, each with unique advantages and disadvantages (Figure 1). Additionally, exosomes have been recently introduced as potential carriers of chemotherapeutic agents for oral cancer treatment. The benefits and drawbacks of each carrier system are summarized.

Figure 1. Different carriers used for oral cancer: (A) polymeric nanoparticles; (B)nanolipids; (C) inorganic nanoparticles; (D) hydrogels.

4. Carriers for Drug Delivery

4.1. Polymeric nanoparticles Advantages

• Biodegradable and biocompatible

• Suitable for controlled and sustained drugs release with increased therapeutic                    efficacy and reduced side effects

Disadvantages

• Difficult to handle due to particle-particle aggregation

• Cytotoxic after internalization into cells

• Not suitable for the release of proteins including antibodies

• Associated with an immune response or local toxicity upon degradation

4.2. Inorganic nanoparticles Advantages

• Target can be site specific by attaching the ligand to the nanoparticle (e.g., magnetic nanoparticles)

• Higher photostability compared to organic dyes

Disadvantages

• Toxicity

• Limited effective delivery due to limited penetration depth for photothermal therapy

• Cannot deliver biomacromolecules (e.g., proteins)

4.3. Nano lipids Advantages

• Highly stable

• Provide controlled release of drugs to protect them from chemical degradation

• Encapsulate and deliver drugs with low aqueous solubility

• Able to penetrate deeply into tumors

• Suitable for local delivery of anticancer drugs

Disadvantages

• Crystalline structure provides limited space to accommodate drugs

• Solid lipid nanoparticles (SLNs) show initial burst drug release

• Aggregation or gelling of nanostructured lipid carriers (NLCs) during storage

• Associated with immune response

4.4. Hydrogels Advantages

• Injectable to a specific site

• Do not dissolve in water at physiological temperature and pH

• Maintain their structural integrity and elasticity even after retaining large amounts of water

• High drug loading capacity

• Ability to deliver hydrophilic and hydrophobic drugs

Disadvantages

• Poor mechanical properties

• Difficult to handle

• Expensive

• Initial burst

1. 1. 2. Materials And Strategies Used In Cancer Therapy

Several innovative methods of drug delivery are being used in cancer treatment. A wide range of nanoscale compounds basedon synthetic polymers, proteins, lipids, and organic and inorganic particles have been used for the formulation of cancer therapeutics. In comparison to direct injection of unmodified chemo-drugs, encapsulation of drugs in a carrier provides several benefits, including protection against degradation in blood, improved drug solubility, improved drug stability, targeted drug delivery, reduced toxic side effects and better pharmacokinetic and pharmacodynamic drug characteristics. So far, an impressive library of different drug delivery vehicles has been developed with different sizes, architectures, and surface physicochemical properties with targeting strategies (Scheme 1). Table 1 lists some examples of drug delivery systems that have either been approved or are in clinical or preclinical development stages.

5.1. Nanocarriers For Drug Delivery

Nanomedicine is a rapidly developing area that is revolutionizing cancer diagnosis and therapy. Nanoparticles have unique biological properties given their small size (diameter within 1–100 nm) and large surface area to volume ratio, which allows them to bind, absorb and carryanticancer agents, such as drugs, DNA, RNA, and proteins, along with imaging agents with high efficiency. Nanocarriers used in chemotherapy can be classified into two major types designed for targeted or non-targeted drug delivery: vehicles that use organic molecules as a major building block material and those that use inorganic elements (usually metals) as a core. Organic nanocarriers are comprised of liposomes, lipids, dendrimers, carbon nanotubes, emulsions, and synthetic polymers.

5.2. Inorganic nanocarriers

Inorganic nanocarrier platforms have been intensively investigated for therapeutic and imaging treatments in recent years due to their great advantages, such as large surface area, better drug loading capacity, better bioavailability, lower toxic side effects and controlled drug release, and their tolerance towards most organic solvents, unlike polymer-based nanoparticles. Quantum dots, carbon nanotubes, layered double hydroxides, mesoporous silica and magnetic nanoparticles are commonly used in cancer treatment in various ways. Quantum dots have already been proven to be powerful imaging probes, especially for long- term, multiplexed and quantitative imaging and diagnostics14–16. Zero dimensional (0- D) fluorescent nanoparticles, such as quantum dots (QDs) within the size of 1–10 nm, have emerged as one of the most promising nanoparticles for targeted and traceable drug delivery systems, real-time monitoring of intracellular processes and in vivo molecular imaging due to their unique physicochemical properties, such as uniform size, large surface-to-volume ratio, biocompatibility, highly tunable photoluminescence property, improved signal brightness, resistance against photobleaching and multicolor fluorescence imaging and detection17. However, the main challenge with QDs in biological applications is their hydrophobic nature, high tendency of aggregation and non-specific adsorption18,19. QD surfaces are usually coated with polar species and/or monolayer or multilayer ligand shells to make them water soluble and to enhance their bioactivity20. This type of coating also helps in the development of multifunctional QDs, where imaging contrast agents and small molecular hydrophobic drugs can be embedded between the inorganic core and the amphiphilic polymer coating layer while hydrophilic therapeutic agents

5.3. Combinational (Polymeric-Inorganic) Nanoparticles

Combinational drug therapy is known for enhanced therapeutic effects. Targeted drug delivery provides better therapeutic effect with less toxicity. Quinacrine (QC) is an anticancer drug that also acts as an antimalarial agent; it has demonstrated therapeutic effects in breast, lung, colon, and renal cell carcinoma. In spite of these favorable effects, QC clinical uses are limited because of its low bioavailability and several side effects, such as skin rash and pigmentation, and immunological side effects [116]. Inorganic silver-based nanoparticles (AgNPs) are also promising as anticancer agents because they can cause tumor cell apoptosis. Combinatorial strategies have been utilized to overcome AgNP's drawback of toxicity to normal cells at high concentrations, which led to the improved anticancer activity of AgNPs [100,116]. Satapathy et al. synthesized highly stable PLGA based quinacrine (QC)–silver hybrid nanoparticles (QAgNP) through an oil-in-water emulsion solvent evaporation method. TEM analysis revealed the size and morphology of QAgNP with size ranging between 50–100 nm. DLS gave average particle size of 382.4 ± 0.11 nm with a positive zeta potential of 0.523 ± 0.09 mV [111]. These nanoparticles were permitted to interact with different oral cancer cell lines and OSCC- derived stem cells and assessed for their antitumor activity. PLGA/quinacrine/silver nanoparticles exhibited high cytotoxicity against cancer cells with enhanced capability to kill particularly the OSCC-derived stem cells. The research also validated that PLGA/quinacrine/silver nanoparticles not only suppressed proliferation of OSCC but also inhibited neo-angiogenesis, indicating that such a hybrid nanoparticle drug delivery system can serve as a valuable platform for treating OSCC [100,116].

5.4. Nonlipids

Polymeric nanoparticles' cytotoxicity, by low internalization into cancer cells, limits their therapeutic efficacy [85,86]. Solid lipid nanoparticles (SLNs) have been able to overcome this issue as they are able to penetrate into the cancer cells. In addition, their high stability ensures controlled drug release, drug protection from chemical degradation, and they can act as carriers for low aqueous solubility drugs [92,117]. Hence, these nanoparticles appear to be appropriate for the local delivery of drugs and chemopreventive agents [118,119]. Pharmaceutics 2019, 11, 302 12 of 29 A drawback of nanoparticles made from solid lipids is that they possess a crystalline structure which has only a limited space to fit drugs. Nanostructured lipid carriers (NLCs) have been engineered and used in cancer treatment to break this limit. The NLCs are composed of both solid and liquid lipids in a core matrix, thus deforming the crystal structure and creating space for drugs to be trapped in amorphous clusters [120,121]. NLCs thereby solved the problems of low solubility, low bioavailability, and instability of anticancer drugs and therapeutic agents [93,121] Fang et al. recently reported increased bioavailability of curcumin incorporated into nanostructured lipid particles, a new technique for treating OSCC [122]. Other research reported the preparation of nanostructured lipids with other therapeutic agents, including docetaxel and etoposide, which have shown promise in treating oral cancer [123–125].

5.5. Hydrogel-Based Drug Delivery Systems

Hydrogels are 3D mesh matrices of hydrophilic fibers and have a lot of water or biological fluid entrapped. Hydrogels can mimic the soft body tissues and can encapsulate drugs and biomolecules like proteins and genetic material [126]. Based on the mechanism of their gelation, hydrogels are either physical or chemical, depending upon their type. Physical gelation is not always permanent but reversible in nature whereas chemical gelation is reversible in nature because it involves chemical bonds, and therefore leads to permanent or highly stable hydrogels [127–129]. Hydrogels are localized, targeted drug delivery systems and possess some benefits when compared with active and passive targeting by employing nanocarriers [130]. For example, a drawback of nanoparticle-based systems is the rapid elimination from blood circulation due to their small size and renal clearance. Furthermore, the tumor microvascular morphology, which is characterized by enhanced interstitial fluid pressure, leading to low intra-tumoral penetration of the drug-loaded nanocarriers, which consequently leads to reduced therapeutic efficiency [130–133]. Hydrogels, on the other hand, can deliver sustained release of hydrophilic as well as hydrophobic drugs, proteins and other biomolecules regardless of the microvascular system of the tumor, with high drug loading capacity, as high as the solubility of the drug in water [134,135]. Hydrogels can also regulate the delivery of drug for short or long durations (up to several months) by modifying the density of the nanofibers in the hydrogel [136]. Additionally, hydrogels enable co-delivery of multiple drugs with synergistic anti-cancer activities and reduced drug resistance [46,130]. In a study, a thermosensitive physical hydrogel made of poly(ethylene glycol)-poly(ε- caprolactone)-poly(ethylene glycol) (PEG-PCL-PEG, PECE) exhibited excellent potential as an in situ controlled delivery system for suberoylanilide hydroxamic acid (SAHA), a histone deacetylase (HDAC) inhibitor and cisplatin (DDP) combination. Upon intratumorally injected in an OSCC mouse model, the PECE hydrogel sustained the release of the loaded SAHA and DDP for over 14 days, improved therapeutic outcomes, and minimized side effects [137].

5.6. Controlled Drug Delivery Approaches for Cancer

Advanced OSCC treatment options are limited and not optimal. Traditional therapeutic methods (i.e., surgery, chemotherapy, and radiotherapy) affect the well-being and quality of life of patients. Therefore, there is an urgent need for new therapeutic approaches with fewside effects and systemic toxicity. A number of controlled drug delivery and release mechanisms have been developed to reverse the present limitations of the parenteral (intravenous, IV) delivery of chemotherapeutic agents. Such strategies are: the delivery of chemotherapeutics through intra-tumoral injection; local delivery; photo-thermal delivery with drug-loaded nanoparticles; and ultra-sonoporation with microbubbles (Figure 2). These methods are discussed and reviewed hereunder Figure 2. Different controlled drug delivery approaches: (A) Intra-tumoral drug delivery; (B) local drug delivery; (C) photo-thermal therapies combined to drug delivery systems; (D) ultrasound-mediated microbubble.

5.7. Intra-Tumoral Drug Delivery in Oral Cancer

One strategy is local intra-tumoral administration [146,147]. Li et al. created a controlled release system which maximized the combined therapeutic efficacy of two anticancer agents with reduced side effects, utilizing suberoylanilide hydroxamic acid (SAHA) and cisplatin (DDP) loaded PECE hydrogel for the OSCC therapy. Six groups of mice were compared comparatively (1st group received normal saline (NS); 2nd group received blank hydrogel; 3rd with SAHA; the 4th with DDP; the 5th with SAHA-DDP; and the 6th with SAHA-DDP/PECE; the sixth group of mice had the lowest tumor volume without any visible systemic cytotoxicity when compared to other groups at study termination [137]. Intra-tumoral administration of chemotherapeutic agents encapsulated in a hydrogel is viewed as a promising strategy for further investigation of OSCC treatment [137]. Figure 2. Various controlled drug delivery methods: (A) Intra-tumoral drug delivery; (B) local drug delivery; (C) photo-thermal therapies combined to drug delivery systems; (D) ultrasound-mediated microbubble.