Folate Targeting Enables Durable and Specific Antitumor Responses from a Therapeutically Null Tubulysin B Analogue

Endocyte, Inc., West Lafayette, Indiana 47906, USA.
Cancer Research (Impact Factor: 9.33). 01/2009; 68(23):9839-44. DOI: 10.1158/0008-5472.CAN-08-2341
Source: PubMed


The membrane-bound high-affinity folate receptor (FR) is highly expressed on a wide range of primary and metastatic human cancers, such as those originating in ovary, lung, breast, endometrium, kidney, and brain. Because folate-linked conjugates bind to and become internalized within FR-expressing cells (similar to that of free folic acid), we explored the possibility of using the folate ligand to target a potent, semisynthetic analogue of the microtubule inhibitor tubulysin B to FR-enriched tumors. When tested in vitro, a novel folate conjugate, herein referred to as EC0305, was found to specifically inhibit the growth of a panel of FR-positive cell lines (IC50 range, 1-10 nmol/L) in a dose-dependent manner, whereas cells lacking FR expression were unaffected. The potency of EC0305 was also confirmed against a human KB xenograft-nu/nu mouse cancer model. Here, a brief three times per week, 2-week regimen yielded remarkable antitumor activity (100% tumor-free animals) without causing significant weight loss or major organ tissue degeneration. In contrast, antitumor activity was completely abolished in EC0305-treated animals that were co-dosed with an excess of a nontoxic folate-containing analogue, thereby confirming that the antitumor effect of this agent was mediated by FRs. The advantage provided by folate conjugation was further proved by the untargeted free drug, which was found to be completely inactive at both tolerable and highly toxic dose levels. Collectively, these results show that this potent antiproliferative tubulysin compound can be specifically delivered to FR-positive tumors to provide substantial therapeutic benefit using well-tolerable dosing regimens.

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    • "It has been demonstrated that neoplastic tissues have significantly up-regulated levels of serine synthetic enzymes and SHMT, and the increased capacity for serine synthesis in cancer cells was coupled with its preferential utilization for the provision of nucleotide precursors for enhanced growth potential [32]. Based on their vital role in de novo biosynthesis of purines and thymidylate, folate-requiring enzymes have long been considered viable targets for anti-cancer therapy [33-37]. Methotrexate (MTX) is one of the most widely used anti-folate agents in chemotherapy, blocking de novo nucleotide synthesis by depleting reduced THFs, mainly through inhibition of dihydrofolate reductase (DHFR) and thymidylate synthase (TS). "
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    • "Thus, while native tubulysin B–H was unable to produce an antitumor effect due to its high nonspecific toxicity, EC0305 was able to induce tumor regression with no apparent dose-limiting toxicity.88 Furthermore, the IC50 values of EC0305 were three-fold lower than other similar folate-targeted chemotherapeutics, such as folate-mitomycin C and folate-camptothecin.32,61–64,89,91 Recent studies also demonstrated that EC0305 was more effective against M109 (murine lung) and 4T1-C12 (murine mammary) cancer cells compared to folate-desacetylvinblastine monohydrazide (EC145), a folate-targeted drug that has reached phase II clinical trials.88 "
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