Formulation And Evaluation of Controlled-Release Minoxidil Nanogels for Topical Application

Hair loss, clinically termed alopecia, is a pervasive dermatological condition characterized by the partial or complete absence of hair from areas of the body where it typically grows. This condition affects an extraordinarily diverse demographic, impacting individuals across all age groups and genders globally. The consequences of alopecia extend far beyond mere cosmetic concerns, often leading to significant psychological distress, a profound reduction in self- esteem, and a noticeable impairment in overall quality of life [1,2].

1.1.      Types of Alopecia

Alopecia manifests in a wide array of forms, each distinguished by unique underlying causes, patterns of hair loss, and clinical presentations. The most prevalent type, affecting millions worldwide, is androgenetic alopecia (AGA), commonly referred to as male or female pattern baldness. This form is predominantly influenced by a complex interplay of genetic predisposition and the activity of androgen hormones, particularly dihydrotestosterone (DHT) [3]. Beyond AGA, alopecia areata stands out as a distinct autoimmune condition where the body's immune system mistakenly attacks its hair follicles, leading to characteristic patchy, non-scarring hair loss that can range from small, isolated areas to complete scalp or body hair loss [4].

Figure 1: Types of Alopecia

1.2.      Pathophysiology of Hair Loss

The pathophysiology of hair loss is intricate and highly dependent on the specific type of alopecia. In androgenetic alopecia, the central pathological event is the progressive miniaturization of hair follicles. This process is driven by the intracellular conversion of testosterone to the more potent dihydrotestosterone (DHT), catalyzed by the enzyme 5-alpha reductase, predominantly type II, which is highly expressed in scalp hair follicles [5]. DHT exerts its effects by binding to specific androgen receptors present in the cytoplasm of genetically susceptible hair follicles. This binding initiates a cascade of events that leads to a profound alteration in the hair growth cycle: the anagen (growth) phase is progressively shortened, while the telogen (resting) phase is prolonged [6].

Figure 2: Pathophysiology of Hair Loss

1.3.      Nanogels

Nanogels represent a groundbreaking class of nanomaterials that have rapidly emerged as highly promising platforms in the field of advanced drug delivery systems. These sophisticated systems skillfully combine the advantageous characteristics of both traditional hydrogels and nanoparticles, offering a versatile and robust platform for enhancing therapeutic efficacy, particularly in complex applications like topical drug delivery. Their inherent ability to encapsulate a wide spectrum of therapeutic agents, ranging from small molecules to large biologics, and precisely control their release kinetics makes them an exceptionally attractive solution for overcoming many of the limitations associated with conventional drug formulations [7,8].

Figure 3: Types of Nanogels

1.4.      Controlled Release Drug Delivery Systems

Controlled-release drug delivery systems represent a sophisticated paradigm in pharmaceutical science, meticulously engineered to deliver a therapeutic agent to the body at a predetermined rate over a significantly prolonged period. The fundamental aim of such systems is to maintain optimal and consistent drug concentrations at the specific target site, while simultaneously minimizing undesirable fluctuations in systemic drug levels. This strategy stands in stark contrast to conventional immediate-release formulations, which often lead to a rapid initial surge in drug concentration (the "peak") followed by a rapid decline (the "valley"), potentially resulting in periods of sub-therapeutic efficacy or, conversely, transient toxic concentrations [9,10].

2.    MATERIALS AND METHODS

2.1. Materials

Minoxidil (API) was obtained from Windlas Biotech Ltd.. Chitosan was provided by the department. PVA was purchased from Otto Chemie Pvt. Ltd., and TPP from Fisher Scientific. All other analytical grade reagents were procured from Rankem.

2.2.      Methodology

Minoxidil-loaded nanogels were prepared using a precise and controlled ionic gelation method, a technique well-suited for chitosan-based nanoparticles due to its mild reaction conditions and absence of toxic organic solvents [11,12]. The detailed procedure was as follows:

Chitosan Solution Preparation:

•      A predetermined amount of chitosan (based on the optimized concentration from the BBD) was accurately weighed and slowly dispersed into a 1% (v/v) acetic acid solution. This mixture was then stirred continuously using a magnetic stirrer at room temperature (25 ± 2°C) for 2 hours to ensure complete dissolution and homogeneity of the chitosan solution.

•      Minoxidil Incorporation: The accurately weighed amount of minoxidil (corresponding to the optimized concentration) was then added to the prepared chitosan solution. The mixture was stirred for an additional 30 minutes to ensure uniform dispersion and dissolution of the drug within the polymeric solution.

•      TPP Solution Preparation: Separately, the optimized concentration of TPP was accurately weighed and dissolved in a specific volume of deionized water to create the cross- linking solution.

•      Nanogel Formation (Ionic Gelation): The TPP solution was carefully added drop- wise to the minoxidil-chitosan solution using a peristaltic pump (Masterflex L/S, Cole-Parmer, USA) at a controlled rate (optimized to 0.5 mL/min, as determined in process optimization) under continuous moderate magnetic stirring (optimized to 500 rpm) at room temperature. The immediate interaction between the positively charged amino groups of chitosan and the negatively charged phosphate groups of TPP resulted in spontaneous ionic cross-linking and the formation of discrete nanogel particles. Stirring was continued for an additional 30 minutes after complete TPP addition to allow for stabilization of the nanogel structure.

•      Separation and Washing: The resulting milky nanogel suspension was then subjected to high-speed centrifugation (Sigma 3K30, Germany) at 15,000 rpm for 30 minutes at 4°C topellet the formed nanogels. The supernatant, containing any unentrapped minoxidil and unreacted TPP, was discarded. The nanogel pellet was then washed twice with cold deionized water by resuspension and centrifugation to ensure complete removal of free drug and impurities.

•      Storage: The washed nanogel pellet was finally re-suspended in a minimal volume of deionized water to form a concentrated nanogel dispersion and stored at 4°C for further characterization and studies.

3.         RESULT AND DISCUSSION

3.1.         Formulation Development and Optimization Results

3.2.      Effect of Variables on Nanogel Properties:

The Box-Behnken Design (BBD) effectively elucidated the influence of chitosan concentration (X1), TPP concentration (X2), and minoxidil concentration (X3) on the critical quality attributes of the nanogels (particle size, PDI, EE%, viscosity). The ANOVA results indicated that all three independent variables significantly influenced the particle size and entrapment efficiency (p < 0.05).

•           Particle Size and PDI: An increase in chitosan concentration (X1) generally led to an increase in particle size, likely due to increased polymer chain entanglement and cross-linking density (Figure 4). Conversely, increasing TPP concentration (X2) initially decreased particle size by providing more cross-linking sites, but very high TPP levels sometimes led to aggregation and larger particles due to excessive cross-linking or charge neutralization, consistent with reported literature on ionic gelation. Minoxidil concentration (X3) showed a less pronounced but still significant effect, with higher drug loads sometimes leading to slightly larger particles due to the bulk effect of the drug. PDI values were generally low (< 0.3), indicating a narrow size distribution, but extreme concentrations of X1 or X2 could increase PDI.

 •          Entrapment Efficiency (EE%): EE% was directly proportional to chitosan concentration (X1) (Figure 5), as higher polymer content provided more binding sites and a denser matrix for drug encapsulation. TPP concentration (X2) exhibited an optimal range; insufficient TPP led to poor cross-linking and low EE, while excessive TPP could hinder drug encapsulation due to tight network formation. Minoxidil concentration (X3) had an inverse relationship with EE%, as exceeding the polymer's capacity for encapsulation resulted in lower efficiency.

•           Viscosity: The viscosity of the nanogel dispersion was primarily influenced by chitosan concentration (X1); higher chitosan content resulted in significantly higher viscosity, as expected for polymeric gels. TPP and minoxidil concentrations had less direct impact on overall viscosity but affected particle properties, which indirectly influence the dispersion's rheology.

Figure 4: Response Surface Plot for the Effect of Chitosan and TPP Concentration on Particle Size

Figure 5: Response Surface Plot for the Effect of Chitosan and Minoxidil Concentration on Entrapment Efficiency

3.3.      Optimized Nanogel:

Formulation Based on the desirability function generated by the BBD, the optimized minoxidil nanogel formulation (designated as MN-Opt) was identified. The target criteria for optimization were minimal particle size and PDI, maximal entrapment efficiency, and a

viscosity suitable for topical application. The optimized composition was determined to be Chitosan: [1.0% w/v], TPP: [0.3% w/v], and Minoxidil: [3% w/w of polymer]. This composition provided a balance of desired properties, confirming the effectiveness of the BBD in navigating the complex interplay of formulation variables. The resulting MN-Opt batch demonstrated excellent physical stability during initial observation as shown in the Table 6.3.

Table 1: Different Parameters of 6 Batches

Characterization Results of Optimized Minoxidil Nanogel

Table 2: Physical and Chemical Characterization

Morphological Analysis (SEM Image):

Figure 6: Representative SEM Image of Optimized Minoxidil Nanogel (MN-Opt)

Thermal and Crystalline Properties (DSC/XRD Data): The DSC thermogram of pure minoxidil exhibited a characteristic sharp endothermic peak corresponding to its crystalline nature, whereas the optimized nanogel (MN-Opt) showed peak broadening and reduced intensity, indicating successful drug encapsulation and partial amorphization within the polymeric matrix (as shown in Fig.7).

Figure 7: DSC Thermograms of Minoxidil, Excipients, and MN-Opt

The XRD pattern of pure minoxidil displayed distinct sharp diffraction peaks confirming its crystalline structure, while MN-Opt exhibited diminished peak intensity and reduced crystallinity, suggesting effective incorporation of the drug into the chitosan-based nanogel system (as shown in Fig.8).

Figure 8: X-Ray Diffraction (XRD) patterns of Minoxidil, Excipients, and MN-Opt

In vitro Drug Release Profile

Figure 9: In Vitro Cumulative Percentage Release of Minoxidil from MN-Opt vs.

Conventional Solution (n=3, mean ± SD)

Release Kinetics Interpretation

Table 3: Kinetic Model Fitting for Minoxidil Release from MN-Opt

Figure 10: Zero-Order Figure 11: First-Order Release Kinetic Model

In Vitro Diffusion Across Synthetic Membrane and Skin Deposition

Figure 12: In Vitro Cumulative Percentage Diffusion of Minoxidil from MN-Opt vs.

Conventional Solution across Strat-M® Membrane (n=3, mean ± SD)

Figure 13: Comparison of Minoxidil Deposition in Strat-M® Membrane from MN-Opt vs.

Conventional Solution (n=3, mean ± SD, p<0.01)