Nanoencapsulation of ABT-737 and camptothecin enhances their clinical potential through synergistic antitumor effects and reduction of systemic toxicity

The simultaneous delivery of multiple cancer drugs in combination therapies to achieve optimal therapeutic effects in patients can be challenging. This study investigated whether co-encapsulation of the BH3-mimetic ABT-737 and the topoisomerase I inhibitor camptothecin (CPT) in PEGylated polymeric nanoparticles (NPs) was a viable strategy for overcoming their clinical limitations and to deliver both compounds at optimal ratios. We found that thrombocytopenia induced by exposure to ABT-737 was diminished through its encapsulation in NPs. Similarly, CPT-associated leukopenia and gastrointestinal toxicity were reduced compared with the administration of free CPT. In addition to the reduction of dose-limiting side effects, the co-encapsulation of both anticancer compounds in a single NP produced synergistic induction of apoptosis in both in vitro and in vivo colorectal cancer models. This strategy may widen the therapeutic window of these and other drugs and may enhance the clinical efficacy of synergistic drug combinations.Colorectal cancer is the third most commonly occurring cancer worldwide. Despite advances in understanding the molecular basis of the disease and development of new therapeutic modalities, the 5-year overall survival for patients with late-stage disease remains poor.¹ Although new compounds are continually developed, the lack of efficacy at systemically tolerable doses frequently precludes their success in the clinic. Formulation and delivery strategies that can improve the narrow therapeutic window of such compounds have the potential to make significant advances in treatment regimens.ABT-737 is a small molecule that targets the BH3-binding hydrophobic cleft of antiapoptotic B-cell lymphoma (Bcl) proteins Bcl-2, Bcl-w and Bcl-X(L), which are frequently upregulated in tumors and strongly associated with chemoresistance.² As a single agent, ABT-737 is particularly potent against leukaemia/lymphoma cells and a range of small-cell lung carcinomas, causing complete regression of solid tumors in mouse models.³ Moreover, its orally bioavailable derivative, ABT-263, has shown potential clinical utility in combination therapies as it has been demonstrated to sensitize tumor types, including colorectal cancer cells, to a range of chemotherapies.⁴ However, the clinical evaluation of ABT-263 has revealed that its therapeutic effects are compromised by severe dose-dependent thrombocytopenia.^{5, 6, 7} Platelets normally survive in circulation for several days by maintaining elevated levels of Bcl-X(L), the inhibition of which by ABT-263/ABT-737 results in premature initiation of platelet apoptosis.^8,9 In spite of these issues, ABT-263 is still under clinical investigation in combination with frontline cytotoxic chemotherapies such as irinotecan and other therapies (Supplementary Table S1).Camptothecin (CPT) is a potent topoisomerase-I inhibitor; however, it is poorly soluble and rapidly hydrolyzed from a lactone to an inactive carboxylic acid in an aqueous environment, thus limiting its clinical applicability. Consequently, this has led to the development of derivatives such as irinotecan.¹⁰ Although irinotecan overcomes some of the pharmacokinetic problems associated with CPT, it has reduced inhibitory activity and still exhibits systemic toxicities such as neutropenia and dose-limiting diarrhea owing to the damage of the intestinal mucosa.¹¹Pharmaceutical formulations that improve the safety profile of potent anticancer drugs such as ABT-737/263 and CPT are urgently required. Drug-loaded nanotherapeutics are finding increasing application in a range of solid tumors, best exemplified by Doxil, a liposomal preparation of doxorubicin that reduces drug-associated myocardiotoxicity.¹² Such drug carriers can not only diminish adverse effects, but also simultaneously enhance tumor localization through the ability of nanosized particles to penetrate defective endothelial junctions in the tumor neovasculature – a phenomenon known as the enhanced permeability and retention (EPR) effect.¹³ Of note, a polymeric formulation of CPT, CRLX101, has been clinically evaluated, and it was demonstrated that the improved pharmacokinetics and tumor penetration of nanoparticle (NP)-based carriers observed in mouse models were maintained in patients,¹⁴ highlighting the therapeutic potential that new formulations hold for this parental molecule.Previously, we developed polymeric NPs encapsulating a range of drug types, formulated in FDA-approved poly-lactide-co-glycolide acid (PLGA).^{15, 16, 17} In the current investigation, we wished to examine the application of both CPT and ABT-737 in colorectal cancer models and determine if a nanodelivery system could be employed to elicit the synergistic efficacies of these agents. We show that nanoencapsulation of ABT-737 reduces thrombocytopenic effects, whereas nanoencapsulation of CPT inhibits its cytotoxic effects towards white blood cells and the gastrointestinal (GI) tract. Both drugs were successfully combined in a single NP formulation to elicit synergistic effects against colorectal cancer cells in vitro and in vivo; this highlights the potential of this approach and similar formulations for widening narrow therapeutic windows for the treatment of colorectal and other cancers with rationally selected drug combinations delivered at pre-determined synergistic concentrations.