Abstract: Camptothecin (CPT) and its highly potent derivative 10-Hydroxycamptothecin (HCPT) are natural plant alkaloids renowned for their robust antitumor activity. However, their clinical application is severely hindered by poor aqueous solubility, low bioavailability, and the inherent instability of their pharmacologically active lactone ring. To overcome these barriers, extensive research has been directed toward the development of advanced nanomedicine and drug delivery systems. This review comprehensively examines the recent progress in CPT and HCPT nano-formulations, including organic nanoparticles (such as PEG, PLGA, liposomes, and dendrimers), inorganic nanoparticles (such as gold, silica, and magnetic materials), and carrier-free or prodrug-based nanosystems. These delivery platforms not only protect the active compound and improve its pharmacokinetic profile but also enable targeted delivery and stimuli-responsive release, thereby maximizing therapeutic efficacy while minimizing off-target toxicity. Despite promising preclinical results, challenges such as the over-reliance on the enhanced permeability and retention (EPR) effect and manufacturing complexities remain. Future perspectives emphasize the need for deeper mechanistic understanding and the optimization of tumor penetration to facilitate the successful clinical translation of CPT-based nanomedicines.
1. Introduction
Camptothecin (CPT) and its derivatives, most notably 10-Hydroxycamptothecin (HCPT), are natural alkaloids isolated from the plant Camptotheca. These compounds have demonstrated potent apoptotic and antitumor abilities against a wide variety of cancer cells [1]. HCPT, in particular, is recognized as the most potent anticancer compound among more than 20 monomers isolated from the plant, exhibiting an anticancer activity equivalent to 30 times that of CPT [1]. Despite their high efficiency and broad anticancer spectrum, the clinical application of CPT and HCPT is severely limited by their poor water solubility, low bioavailability, and the instability of their active lactone ring [1].
To overcome these pharmacological challenges, the incorporation of CPT and HCPT into nano-sized drug delivery systems has emerged as a highly promising strategy. Nanomedicine offers the ability to significantly improve the stability and water solubility of these hydrophobic drugs, prolong their circulation time in vivo, and enhance their slow and controlled release [1]. Furthermore, nano-drug delivery systems can effectively enhance the in vivo distribution of chemotherapeutic agents, overcoming the lack of specificity seen in conventional therapies and improving the balance between efficacy and systemic toxicity [1].
2. Pharmacological Activity
CPT and HCPT exhibit a broad anticancer spectrum and are clinically relevant for the treatment of colorectal cancer, liver cancer, gastric cancer, ovarian cancer, leukemia, head and neck tumors, and glioblastoma [1] [3]. The pharmacological activity of these compounds is vastly amplified when delivered via nanocarriers.
For instance, in colorectal cancer models, PEGylated G5 PAMAM dendrimers loaded with CPT and surface-grafted with AS1411 anti-nucleolin aptamers (Apt-PEG-PAMAM-CPT) demonstrated potent antiproliferation activity against nucleolin-positive HT29 and C26 cells. In vivo studies showed that this targeted nanodevice significantly improved mouse survival rates and exhibited superior tumor growth inhibition compared to free CPT [2]. In the context of glioblastoma, a reactive oxygen species (ROS)-responsive prodrug of CPT (CPT-TEG-ALA) assembled into a nanoprodrug inhibited 80% of tumor growth in a subcutaneous U87 glioma xenograft mouse model and significantly extended median survival time in intracranial models [3].
Additionally, various organic and inorganic HCPT-loaded nanoparticles—such as PEG-based micelles, PLGA nanoparticles, liposomes, and gold (Au) nanoparticles—have consistently shown superior in vitro cytotoxicity and in vivo tumor elimination capabilities compared to free HCPT injections, often achieving complete tumor ablation when combined with photothermal or photodynamic therapies [1].
3. Molecular Mechanism of Action
At the molecular level, CPT and HCPT exert their potent antitumor activity by targeting and inhibiting intracellular topoisomerase I, an essential enzyme for DNA replication and transcription, thereby inducing DNA damage and cellular apoptosis [1]. However, the efficacy of these drugs is heavily dependent on the mechanism of action of their delivery systems, which dictate how the drug reaches the intracellular target.
Passive and Active Targeting: Nanocarriers primarily utilize the enhanced permeability and retention (EPR) effect for passive accumulation in tumor tissues [1]. To improve specificity, active targeting mechanisms are employed. For example, AS1411 aptamer-conjugated dendrimers specifically bind to nucleolin proteins overexpressed on colorectal cancer cells, facilitating receptor-mediated endocytosis [2]. Similarly, folic acid (FA) modifications target folate receptors, and hyaluronic acid (HA) modifications target CD44 receptors on various cancer cells [1].
Stimuli-Responsive Release: Advanced nanomedicines are designed to release CPT/HCPT in response to specific tumor microenvironment triggers. These include pH-responsive systems that degrade in acidic tumor environments, redox/GSH-sensitive micelles, and ROS-responsive prodrugs [1] [3]. External stimuli such as low-intensity focused ultrasound (LIFU), near-infrared (NIR) laser irradiation for photothermal therapy (PTT), and magnetic fields are also utilized to trigger rapid, localized drug release and enhance tissue penetration [1].
4. Structure-Activity Relationship (SAR)
The structural integrity of CPT and HCPT is paramount to their biological activity. The pharmacologically active form relies on an intact lactone ring, which is highly unstable and prone to hydrolysis in physiological conditions. Encapsulation within hydrophobic cores of nanocarriers (such as lipid-polymer hybrid nanoparticles, solid lipid nanoparticles, or dendrimer cavities) shields the lactone ring from the aqueous environment, thereby preserving its active form until it reaches the target site [1] [2].
The architecture of the delivery vehicle also dictates the structure-activity relationship of the formulated drug. For instance, the modification of dendrimer peripheries with aptamers decreases the diffusion rate of CPT from the dendrimer cavities, resulting in a more sustained and controlled release profile [2]. Furthermore, the physical shape and aspect ratio (AR) of the nanocarriers significantly influence cellular internalization. Studies have shown that HCPT-loaded nanorods (NRs) exhibit stronger shape-dependent effects on cell internalization and higher cytotoxicity compared to spherical nanospheres. Similarly, shorter nanocrystals with specific aspect ratios are more readily captured by the liver, affecting their tumor uptake efficiency [1]. In prodrug designs, conjugating CPT with molecules like alpha-lipoic acid (ALA) and tetra(ethylene glycol) (TEG) allows for spontaneous nano-emulsification, greatly enhancing drug-loading capacity while reducing the need for inert carrier materials [3].
5. Current Limitations
Despite the unprecedented advances in biomedical nanotechnology, the clinical translation of CPT and HCPT nanomedicines remains highly limited. Currently, no HCPT nanomedicines have been approved by the FDA for clinical use [1]. Several critical limitations hinder their progress:
First, the reliance on the EPR effect is increasingly challenged. While the EPR effect is well-documented in small animal models, it exhibits significant heterogeneity in human tumors, leading to inefficient accumulation and low tumor penetration rates of nanomedicines in clinical settings [1]. Second, different nanocarriers present specific drawbacks: PEGylated systems can limit cellular uptake; PLGA and PLA nanoparticles often suffer from an initial uncontrolled burst release; lipid-based nanoparticles can be unstable and prone to membrane leakage; and dendrimers are time-consuming to synthesize and may exhibit poor biodegradability [1]. Third, inorganic nanoparticles (such as Au, Cu, and Pt) carry potential risks of long-term cytotoxicity and non-degradability [1]. Finally, while carrier-free and prodrug-based nanosystems offer high drug loading, their self-assembly processes can be unpredictable, difficult to control, and prone to precipitation [1] [3].
6. Future Perspectives
To bridge the gap between preclinical success and clinical application, future research must pivot towards a deeper understanding of the mechanisms by which nanomedicines exert their effects beyond the traditional EPR paradigm [1]. Strategies to enhance vascular permeability and optimize nanoparticle tumor penetration will be critical.
The development of multifunctional, stimuli-responsive, and actively targeted systems—such as aptamer-dendrimer bioconjugates and ROS-responsive prodrugs—holds great promise for improving specificity and reducing off-target effects [2] [3]. Additionally, exploring carrier-free nanodrugs and prodrug self-assemblies that minimize the use of inert materials could mitigate toxicity concerns associated with synthetic carriers [3]. Ultimately, ensuring the reproducibility, scalability, and long-term biosafety of these sophisticated nano-formulations will be essential to developing CPT and HCPT nanomedicines with high clinical translation rates, bringing new hope to cancer therapy [1].
