Specific RITA modification produces hyperselective cytotoxicity while maintaining in vivo antitumor efficacy

The preclinical antitumor agent RITA (2,5-bis[5-hydroxymethyl-2-thienyl] furan, NSC 652287), an analog of the natural product a-terthiophene, failed during the development phase due to acute pulmonary toxicity in animal models. A series of synthetic modifications to RITA's heterocyclic scaffold resulted in activity ranging from broadly cytotoxic to highly selective. In the National Cancer Institute 60-cell line screen, these "hyperselective" agents (e.g., imatinib) are rare. A Selectivity Index (SI) was developed to quantify this desirable feature, which is 20 for imatinib, while RITA's SI is only 0.10. One of the described hyperselective RITA analogs (SI = 7.9) completely lost activity in the presence of a known SULT1A1 inhibitor. This result, coupled with previous evidence that RITA is a SULT1A1 substrate, suggests that carbinol modification by a sulfate leaving group and subsequent formation of a reactive carbocation may explain RITA's broad cytotoxicity. While SULT1A1 expression is required for susceptibility, hyperselective analogs exhibited reduced association of activity with SULT1A1 mRNA expression compared to RITA, apparently requiring some additional target(s). In support of this hypothesis, there is a strong correlation (P < 0.01, r = 0.95) between quantum mechanically-calculated energy barriers for carbocation formation from sulfonated analogs and SI, indicating that hyperselective RITA analogs generate reactive carbocations less readily after sulfate activation. Importantly, narrowing the cytotoxicity profile of RITA did not eliminate its analogs' in vivo antitumor activity, as several new hyperselective agents, NSC 773097 (1), 773392 (2), and 782846 (6), displayed impressive activity against A498 xenografts in mice. Copyright ©2019, American Association for Cancer Research.

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