10-Hydroxycamptothecin (HCPT) is a DNA inhibitor of topoisomerase I and exerts antitumor activities against various types of cancer. However, reversible conversion from a pharmacologically active lactone form to an inactive carboxylate form of HCPT and poor water solubility hamper its clinical applications. To overcome these shortcomings, we designed a fine self-microemulsifying drug delivery system (SMEDDS) for HCPT to effectively protect HCPT in its active lactone form as well as improving dissolution rates. A formulation of HCPT-SMEDDS that contained ethyl oleate, D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS), and polyethylene glycol 400 (PEG400) was optimized by using the central composite design and response surface methodology. Following 1:100 aqueous dilution of the optimized HCPT-SMEDDS, the droplet size of resulting microemulsions was 25.6 ± 0.7 nm, and the zeta potential was - 15.2 ± 0.4 mV. The optimized HCPT-SMEDDS appeared to stabilize the lactone moiety of HCPT with 73.6% being present in the pharmacologically active lactone forms in simulated intestinal fluid, but only 45.7% for free HCPT. Furthermore, the physically stable formulation showed the active lactone form predominated in HCPT-SMEDDS (> 95%) for 6 months under the accelerated storage condition. Meanwhile, the optimized SMEDDS formulation also significantly improved dissolution rates and membrane permeability of the lactone form of HCPT. Therefore, HCPT-SMEDDS involved designing for the ease of manufacture, and provided a potent oral dosage form for preserving its active lactone form as well as enhancing the dissolution rate.
Keywords: chemical stability; reversible conversion; self-microemulsion; topoisomerase I inhibitor.