Stimulus-Responsive Smart Nanocarriers (Light, Ph, Redox): Mechanisms and Cancer Theranostic Applications

Cancer is a broad term for a group of diseases in which abnormal cells grow and divide uncontrollably. Normally, cells in the body grow, divide, and die in a regulated manner. In cancer, these control mechanisms fail, causing cells to multiply excessively and form tumors or spread to other parts of the body.¹Cancer can develop in almost any tissue or organ of the body. Each type of cancer has its own characteristics and behaviour. The disease begins when a cell escapes the normal controls that regulate cell division and starts growing independently. The term "cancer" was first described by Hippocrates about 2,300 years ago. He used the Greek word "Karkinoma" (or Karkinos), which was later translated into the Latin word "Cancer."

Around 10 million people are diagnosed with cancer every year, and more than 6 million deaths occur annually worldwide. Cancer is common in all countries, but rates are higher in developed countries due to smoking and unhealthy lifestyles.

Definition: “Cancer refers to any one of a large number of diseases characterized by the development of abnormal cells that divide uncontrollably and have the ability to infiltrate and destroy normal body tissue”. cancer is caused by the changes to DNA. most of the cancer causing dna changes occurs in section of DNA called genes. these changes are also called genetic changes.

Some Genes Involved in Human Cancer

Proto-oncogenes are normal genes that produce proteins responsible for stimulating cell growth and division. When these genes undergo mutations, they become oncogenes. Oncogenes produce overactive proteins that continuously signal cells to divide, leading to excessive and uncontrolled cell proliferation.¹²

Tumor suppressor genes are genes that produce proteins that slow down or stop cell division. They act as the body's natural brakes on cell growth. If these genes are mutated, the proteins may become inactive, causing the loss of control over cell division and allowing cells to grow uncontrollably.³

Causes of Cancer

Causes of Cancer: A Macromolecular Investigation

Cancer is a multifactorial disease characterized by uncontrolled cell proliferation resulting from cumulative alterations in cellular macromolecules, including DNA, RNA, proteins, lipids, and carbohydrates. These molecular abnormalities disrupt normal cellular homeostasis, leading to malignant transformation, tumor progression, and metastasis.

DNA Damage and Genetic Mutations

Genomic instability is considered a hallmark of cancer development. Exposure to carcinogenic agents such as tobacco smoke, ultraviolet radiation, ionizing radiation, environmental pollutants, and chemical mutagens can induce DNA damage. These alterations include point mutations, deletions, insertions, chromosomal rearrangements, and gene amplifications. Such mutations may activate oncogenes while simultaneously inactivating tumor suppressor genes, resulting in uncontrolled cellular proliferation and impaired DNA repair mechanisms.

Proto-oncogenes such as RAS, MYC, and HER2 regulate normal cellular growth and differentiation. However, their mutation or overexpression converts them into oncogenes that continuously stimulate cell division. Conversely, tumor suppressor genes including TP53, RB1, BRCA1, and BRCA2 normally inhibit cell proliferation and maintain genomic integrity. Loss of their function promotes malignant transformation.

Epigenetic Alterations

In addition to genetic mutations, epigenetic modifications play a critical role in carcinogenesis. These alterations regulate gene expression without changing the DNA sequence. Major epigenetic mechanisms include DNA methylation, histone modification, and chromatin remodeling. Aberrant DNA methylation can silence tumor suppressor genes, whereas abnormal histone modifications may activate oncogenic pathways, thereby facilitating tumor initiation and progression.

RNA Dysregulation

Various RNA molecules contribute significantly to cancer development. Dysregulated expression of messenger RNA (mRNA), microRNA (miRNA), and long non-coding RNA (lncRNA) affects multiple cellular processes, including proliferation, apoptosis, angiogenesis, and metastasis. Overexpression of oncogenic miRNAs and suppression of tumor-suppressive miRNAs promote cancer progression and therapeutic resistance.

Protein Alterations and Abnormal Signaling Pathways

Genetic and epigenetic abnormalities frequently result in the production of dysfunctional proteins. Overexpression of growth factor receptors such as EGFR, HER2, and VEGFR activates signaling pathways that promote cellular proliferation, survival, and angiogenesis. Similarly, dysregulation of apoptotic proteins including Bcl-2, Bax, and Caspases enables cancer cells to evade programmed cell death, thereby supporting tumor growth and survival.

Oxidative Stress and Reactive Oxygen Species (ROS)

Reactive oxygen species (ROS) are generated during normal cellular metabolism however, excessive ROS production contributes significantly to carcinogenesis. Elevated ROS levels induce DNA strand breaks, protein oxidation, and lipid peroxidation, resulting in genomic instability and cellular dysfunction. Persistent oxidative stress accelerates mutation accumulation and promotes tumor progression.

Lipid Alterations and Membrane Dysfunction

Cancer cells exhibit profound alterations in lipid metabolism to support rapid growth and proliferation. Increased lipid synthesis and lipid peroxidation modify membrane structure and function, affecting cellular signaling pathways associated with proliferation, invasion, and metastasis. These metabolic adaptations facilitate tumor survival within hostile microenvironments.

Chronic Inflammation and Tumor Promotion

Chronic inflammation is strongly associated with cancer development. Inflammatory mediators such as tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and nuclear factor-kappa B (NF-κB) promote DNA damage, angiogenesis, and cellular proliferation. Persistent inflammatory signaling creates a favorable environment for tumor initiation and progression.

Tumor Microenvironment

The tumor microenvironment consists of cancer cells, stromal cells, immune cells, extracellular matrix components, and signaling molecules. Distinct characteristics of the tumor microenvironment include acidic pH, elevated glutathione (GSH) levels, hypoxia, and increased ROS production. These unique physiological conditions contribute to tumor progression and provide the scientific basis for the development of pH-responsive, redox-responsive, and ROS-responsive smart nanocarriers for targeted cancer therapy and theranostic applications.^5,6,7

Types of Cancer

Carcinoma: Originates in the skin or epithelial tissues lining internal organs and includes breast, lung, prostate, and colorectal cancers.

Sarcoma: Develops in connective tissues such as bone, muscle, fat, cartilage, and blood vessels.

Leukemia: A cancer of blood-forming tissues that affects blood cells and bone marrow.

Lymphoma: Arises in the lymphatic system, which is part of the body's immune system.

Central Nervous System (CNS) Cancers: Occur in the brain or spinal cord and include tumors such as gliomas and meningiomas.

Multiple Myeloma: A cancer of plasma cells found in the bone marrow.

Melanoma: Develops from melanocytes, the pigment-producing cells of the skin.

Germ Cell Tumors: Originate from reproductive cells that develop into sperm or eggs.

Neuroendocrine Tumors: Arise from hormone-producing neuroendocrine cells located throughout the body.^8,9,10,11

Mechanism of Cancer

Cancer develops when normal cells acquire genetic mutations that interfere with the normal regulation of cell growth, division, and survival. These mutations can activate oncogenes, which promote cell proliferation, and inactivate tumor suppressor genes, which normally control cell growth and repair damaged DNA. As a result, cells begin to divide uncontrollably and evade normal regulatory mechanisms. The abnormal cells continue to proliferate, resist programmed cell death (apoptosis), and accumulate to form a mass of tissue known as a tumor. With further progression, cancer cells gain the ability to invade surrounding tissues and spread to distant organs through the bloodstream or lymphatic system. This process, known as metastasis, is responsible for the progression of cancer and the development of secondary tumors in other parts of the body.

Conventional therapy

Conventional chemotherapy is one of the most widely used approaches for cancer treatment. In this method, anticancer drugs are administered in their free form and circulate throughout the body via the bloodstream. Although these drugs can kill cancer cells, they also affect healthy tissues because of their non-specific distribution. As a result, patients often experience severe side effects such as nausea, hair loss, fatigue, bone marrow suppression, and organ toxicity. Furthermore, many chemotherapeutic drugs have poor solubility, limited stability, rapid clearance from the body, and low accumulation at the tumor site, which can reduce treatment effectiveness.

The persistent rise in cancer-related deaths, coupled with the inherent risks and severe side effects of existing treatments, highlights the urgent need for more effective and personalized options.

Fig 1: Conventional therapies in cancer

Fig 2: PH responsive model

Redox-Responsive Polymeric Nanocarriers

Fig 3: redox mechanism¹⁹

Fig 4 light responsive model

Stimuli Type	Nanocarrier	Cargo	Cell Line/ Cancer Model	Main Results
Light-responsive	Gold Nanoparticle-Based Nanocarriers	Anticancer drugs and photothermal agents	HeLa	Generated heat under NIR irradiation, leading to efficient tumor cell destruction

Prospects for the future

Intelligent Nanocarriers That Respond to Light

 Future research will concentrate on enhancing tumor specificity, photothermal and photodynamic efficiency, and systems that respond to near-infrared (NIR) light with deeper tissue penetration. It is anticipated that integration with imaging modalities would facilitate real-time cancer diagnosis and treatment monitoring.

Smart Nanocarriers that Respond to pH

Better selective medication delivery in acidic tumor microenvironments and lower systemic toxicity will be possible thanks to advancements in pH-sensitive materials. Therapeutic outcomes may be improved by targeting and imaging ligands integrated with pH-responsive and multifunctional platforms.

 Nanocarriers that respond to redox signals

 Heightened intracellular glutathione levels are anticipated to be used in future redox-responsive systems for precise drug release. Their efficacy in treating metastatic and resistant tumors may be enhanced by combining them with gene therapy, immunotherapy, and combination treatments.

 Applications of Theranostic Cancer Treatment

 Personalized cancer therapy has a bright future with the creation of multipurpose theranostic nanocarriers that can simultaneously diagnose, deliver medication in a targeted manner, and monitor treatment. Better biocompatibility, scalability, and long-term safety evaluation will be necessary for clinical translation.²⁶

CONCLUSION

Targeted cancer therapy and diagnostics may find creative applications for stimulus-responsive smart nanocarriers. To get regulated drug release, they answer particular inputs like pH, redox state, and light. These nanocarriers lower overall toxicity and improve drug buildup at tumor sites. While redox-responsive carriers use high intracellular glutathione levels, pH-responsive systems leverage the acidic tumor microenvironment. Light-sensitive nanocarriers give exact control over the activation of therapy. Combining diagnostic and therapeutic tasks helps real-time tracking of treatment. These tools help to reduce negative effects and raise treatment success. Multi-stimuli-responsive devices improve treatment results and targeting accuracy even more. Still existing, though, are problems with safety, stability, and high-volume manufacturing. More research is needed to solve these restrictions and increase clinical utility. Nanotechnology advances are projected to hasten their integration into clinical application. Personalized cancer treatment and precision medicine will both benefit much from stimulus-responsive intelligent nanocarriers going forward.

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