Toxicity mechanism-based prodrugs: glutathione-dependent bioactivation as a strategy for anticancer prodrug design

ABSTRACT

Introduction: 6-Mercaptopurine (6-MP) and 6-thioguanine (6-TG), two anticancer drugs, have high systemic toxicity due to a lack of target specificity. Therefore, increasing target selectivity should improve drug safety.

Areas covered: The authors examined the hypothesis that new prodrug designs based upon mechanisms of kidney-selective toxicity of trichloroethylene would reduce systemic toxicity and improve selectivity to kidney and tumor cells. Two approaches specifically were investigated. The first approach was based upon bioactivation of trichloroethylene-cysteine S-conjugate by renal cysteine S-conjugate β-lyases. The prodrugs obtained were kidney-selective but exhibited low turnover rates. The second approach was based on the toxic mechanism of trichloroethylene-cysteine S-conjugate sulfoxide, a Michael acceptor that undergoes rapid addition-elimination reactions with biological thiols.

Expert opinion: Glutathione-dependent Michael addition-elimination reactions appear to be an excellent strategy to design highly efficient anticancer drugs. Targeting glutathione could be a promising approach for the development of anticancer prodrugs because cancer cells usually upregulate glutathione biosynthesis and/or glutathione S-transferases expression.

KEYWORDS:

• 6-Mercaptopurine
• 6-thioguanine
• anticancer prodrug design
• bioactivation
• cysteine S-conjugate β-lyases
• glutathione
• Michael addition
• tumor cell-selective prodrug
• trichloroethylene

Article highlights

• Many conventional chemotherapeutic agents, like 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), lack intrinsic target specificity and thus cause severe side effects due to their systemic toxicity. Increasing their selectivity by the prodrug approach may reduce systemic toxicity. Design of the prodrugs of 6-MP and 6-TG exploits high activities of cysteine S-conjugate β-lyases (β-lyases) and high glutathione (GSH) concentrations in kidney and tumor cells. These approaches were inspired by our studies on mechanisms of kidney-selective toxicity of trichloroethylene (TCE), an environmental pollutant.

• Nephrotoxicity of TCE is attributed to its metabolism via the mercapturic acid pathway to form S-(1,2-dichlorovinyl)-L-cysteine (DCVC). DCVC can either undergo a β-elimination reaction catalyzed by β-lyases to form a reactive thiol, or be oxidized by flavin-containing monooxygenase 3 to form DCVC sulfoxide, a Michael acceptor that can readily react with GSH and cause intracellular GSH depletion.

• Several kidney-selective β-lyase-dependent prodrugs of 6-MP and 6-TG were developed. These prodrugs exhibited renal selectivity and low systemic toxicity in rats. However, low turnover rates limited their potential utility.

• Tumor-selective GSH-dependent Michael acceptor prodrugs of 6-MP and 6-TG were then developed. These prodrugs carried vinyl carboxylic acid- or methyl vinyl ketone-moieties. They reacted with GSH to release the parent drugs and caused GSH depletion. Among them, cis-6-(2-acetylvinylthio)purine (AVTP) and trans-6-(2-acetylvinylthio)guanine (AVTG) exhibited excellent anticancer activities against approximately 50 tumor cell lines from different tissues in the National Cancer Institute’s anticancer drug screen. The two prodrugs delivered more thiopurines to tumor cells in vitro than did 6-MP or 6-TG itself and exhibited similar or higher growth-inhibitory activities in vitro compared to 6-MP or 6-TG. Moreover, they showed less in vivo toxicity in mice than 6-TG after single- and multiple-dose regimens.

• Based on the success in the design of AVTP and AVTG, it is proposed that targeting GSH has great potential for development of prodrugs that could have utility against tumor cells resistant to conventional chemotherapy.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Additional information

Funding

This work was funded by the National Institutes of Health via grants GM40375, R01 DK044295 and R01 ES06841 and via a Biomedical Research Support Grant from the University of Wisconsin-Madison.