Abstract C223: BACPTDP: A water-soluble camptothecin pro-drug that exploits the tumor pH gradient for enhanced activity in hypoxic/acidic tumors
Authors: Adams David J. , Waud William R. , Wani Mansukh C. , Manikumar Govindarajan , Flowers James L. , Morgan Lee Roy 
Published: June 2012

Abstract
Hypoxia is a common feature of solid tumors. Related up-regulation of hypoxia-inducing factor-1 (HIF-1) occurs in the majority of primary malignant tumors and in two-thirds of metastases, while most normal tissues are negative. HIF-1 induces the glycolytic phenotype, which creates an acidic extracellular microenvironment (the Warburg Effect). Since tumor cells maintain a normal intracellular pH, the glycolytic phenotype also creates a cellular pH gradient such that drugs that are weak acids are selectively taken up and retained in tumors, while those that are weak bases are excluded. Consequently, one approach to improve selectivity of chemotherapy is to design pH-sensitive antitumor drugs. Camptothecin (CPT) is unique in this regard, since it contains an E ring lactone that opens to form the inactive hydroxy acid at physiological pH, but remains closed to maintain the active species under acidic conditions. Moreover, camptothecins have recently been shown to directly inhibit HIF-1 in addition to the traditional genotoxic target, topoisomerase I. Accordingly, we screened for CPT analogs with enhanced activity at acidic pH and discovered 7-butyl-10-amino-camptothecin (BACPT), an analog that was 10 to 20-fold more active against breast cancer cell lines cultured at pH 6.8 vs 7.4 (Adams et al., Cancer Chemother Pharmacol, 57:145, 2006). We now report that BACPT has superior antiproliferative activity compared to established drugs in monolayer culture models for human neuroblastoma and pancreatic carcinoma and in 3-dimensional histoculture models of colon and primary ovarian cancer. Esterification of BACPT at the 20S position with β-alanine-lysine dipeptide produced a highly water soluble pro-drug, BACPTDP, suitable for in vivo use. BACPTDP displayed antitumor activity in murine xenograft flank models for human colon (HT29), ovarian (SK-OV-3), pancreatic (Panc-1), glioma (SF-295) and non-small cell lung (NCI-H460) cancers. Activity of BACPTDP was comparable to irinotecan in HT29, SF-295 and NCI-H460, significantly greater in SK-OV-3 and remarkably superior in the Panc-1 model, where complete regressions were observed (T-C = 60 d; 2/6 LTS vs T-C = 14 d; 0/6 LTS for irinotecan). Of note, Panc-1 models advanced aggressive disease, since it is poorly differentiated, expresses mutated p53 and K-ras, the c-Src oncogene and is relatively insensitive to gemcitabine and other chemotherapeutic drugs. Based on these data, we evaluated combinations of BACPT with gemcitabine, the standard of care chemotherapy for pancreatic cancer, against Panc-1 cells in vitro. Median effect analysis revealed additive to synergistic interactions that were independent of drug ratio and optimal when gemcitabine was administered 24 h prior to BACPT. Minimal drug antagonism was seen.
Conclusion: BACPTDP is a water-soluble camptothecin pro-drug with defined molecular mechanism(s) that can exploit solid tumor physiology for improved selectivity and activity against multiple tumor types with particular promise for pancreatic cancinoma. Supported by SBIR/NCI grant CA125871.
Citation Information: Mol Cancer Ther 2009;8(12 Suppl):C223.