In vitro characterization of the human biotransformation of marine derived anti-cancer drugs

Abstract

Cancer is the second cause of death in The Netherlands. Although the treatment options over the past few decades have substantially improved, the cure rate for patients with advanced cancer remains low. In addition, hopefully new therapies will induce less severe side effects compared to the present therapies. Overall, new anti cancer drugs are still very much needed to improve treatment outcome of patients. Many active cytotoxic agents originate from natural resources, mainly plants (e.g. paclitaxel originates from the Taxus tree). The search for marine compounds started 50 years ago and new compounds isolated from various marine sources have been described as anti-cancer agent and some have already entered clinical studies. In this thesis, three novel marine derived anti cancer drugs were investigated, i.e. thiocoraline, aplidine, and ET-743. Thiocoraline is derived form the Micromonospora marina, which grows in the Mozambique strait. Aplidine originates from the Mediterranean tunicate Aplidium albicans and ET 743 from the Ecteinascidia turbinata from the Caribbean. All three anti-cancer drugs showed promising activity against various tumor types in preclinical studies. Aplidine and ET-743 have already entered clinical trials and thiocoraline will be investigated in patients in the near future. Little is known about the biotransformation of thiocoraline, aplidine, and ET 743 in the human body. Information on the enzymes involved and the knowledge of the patient enzyme activity may help to improve the treatment of the individual patient and reduction of side effects. Therefore, it is important to elucidate the involvement of the different enzymes with the biotransformation of the anti-cancer drugs thiocoraline, aplidine, and ET 743. The research is focused on the elucidation of the human biotransformation pathways of these drugs. Furthermore, the toxicity of the metabolites was investigated to elucidate if bioactivation or bioinactivation occurred. The cytochrome P450 enzyme 3A4 was found to be very important in the biotransformation of thiocoraline and CYP2C8 was involved to a lesser extent. UGT1A1 and 1A9 were involved in the conjugation of thiocoraline. No involvement of other phase II enzymes was found. Furthermore, the metabolites formed from the CYP reaction were further conjugated by the phase II enzymes UGT, SULT, and GST. Aplidine was mainly metabolized by CYP3A4, but also by CYP2A6, 2E1, and 4A11. Four metabolites were identified from which three were specifically formed by CYP3A4 and one by CYP2A6. Only the UGTs 1A3 and 1A9 were involved in the direct conjugation of aplidine. The metabolites formed by CYP were further conjugated by UGT, SULT, and GST. Cell culture experiments showed that the aplidine metabolites were less toxic compared to aplidine, thus aplidine was bioinactivated by these enzymes. ET-743 was also mainly metabolized by CYP3A4 and a number of other CYPs played a minor role, namely CYP2C9, 2C19, 2D6, and 2E1. In addition, ET-743 was conjugated by GST and UGT. The metabolites formed by cytochrome P450 were less toxic compared to ET-743 itself in cell culture experiments, thus ET-743 was bioinactivated by the CYPs involved in its biotransformation. The studies performed to unravel the biotransformation of the here investigated marine derived anti-cancer agents may help to improve clinical evaluation of anti cancer activity and safety of these drugs.