Preparation of Enzyme-Activated Thapsigargin Prodrugs by Solid-Phase Synthesis

Abstract

Since cells in solid tumors divide less rapidly than cells in the bone marrow or cells of the immune system, mitotic inhibitors often cause severe side effects when used for treatment of diseases like prostate cancer and breast cancer. One approach to overcome this problem involves attempts at developing drugs based on general cytotoxins, like calicheamicin and thapsigargin, which kill cells at all phases of the cell cycle. However, such toxins can only be used when efficient targeting to the malignant tissue is possible. In the case of thapsigargin, selectivity for tumor-associated cells is achieved by conjugating the drug to a peptide that is only cleaved in the vicinity of tumors to release the cytotoxic drug or an analog with retained activity. Solid-phase synthesis protocols were developed for preparation of three already validated prodrugs of thapsigargin: one prodrug cleavable by human kallikrein 2, one prodrug cleavable by prostate-specific antigen, and one prodrug cleavable by prostate-specific membrane antigen.

Large Scale Conversion of Trilobolide into the Payload of Mipsagargin: 8-O-(12-Aminododecanoyl)-8-O-Debutanoylthapsigargin

“…Finally, the cleavage of the Boc group was performed (Section 2.2.9) according to standard procedures [18], i.e., by using a solution of TFA in DCM (Scheme 1g). The lability of the free amine group present within 12ADT (7) makes us suggest that only the Boc-protected derivative Boc-12ADT (6) is stored.…”

“…Three peptides that are fast cleaved by the enzymes that are characteristic of prostatic cancer cell lines are GKAFRRL, HSSKLQL, and βDγEγEγEγE. GKAFRRL is cleaved by human kallikrein-related peptidase 2 (KLK2; formerly human kallikrein 2 or hK2), HSSKLQL by prostate-specific antigen (PSA), and βDγEγEγEγE by prostate-specific membrane antigen (PSMA = glutamate carboxypeptidase II = N-acetyl-L-aspartyl-L-glutamate peptidase I) [18]. These peptides are cleaved only to a limited extent by other proteases present in the human body.…”

Targeting Toxins toward Tumors

“…The C-terminal βAsp residue was introduced in order to make the prodrug a substrate for PSMA [ 49 ]. This prodrug, which has been prepared by solid-phase synthesis [ 55 ], was cleaved rapidly in tumors to release the βAsp derivative, which slowly was cleaved to provide the free mipsagargin. A solid-phase synthesis of these guaianolide prodrugs were developed [ 55 ].…”

Development of Potential Antitumor Agents from the Scaffolds of Plant-Derived Terpenoid Lactones

“…Thus, a promising TPG prodrug, mipsagargin (3b, also named G-202), was produced by solid-phase synthesis protocols (Figure 7), which is based on the hypothesis that the extracellular prostate-specific membrane antigen (PSMA) may be used to cleave the masking peptide to deliver mipsagargin into tumors specifically. 81 The maximum tolerated dose (MTD), the dose-limiting toxicities (DLTs), the pharmacokinetics, and the recommended phase II dose of mipsagargin (3b) have been determined in a phase I clinical trial study. Altogether, 44 patients (28 patients in the dose escalation phase and 16 patients in an expansion cohort), with different types of locally advanced or metastatic solid tumors, were treated (i.v.)…”