Targeted Tubulysin B Hydrazide Conjugate for the Treatment of Luteinizing Hormone-Releasing Hormone Receptor-Positive Cancers

The targeted delivery of chemotherapeutic agents to receptors that are overexpressed on cancer cells has become an attractive strategy to concentrate drugs in cancer cells while avoiding uptake by healthy cells. Luteinizing hormone-releasing hormone receptor (LHRH-R) has attracted considerable interest for this application, since LHRH-R is upregulated in ∼86% of prostate cancers, ∼80% of endometrial cancers, ∼80% of ovarian cancers, and ∼50% of breast cancers, but virtually absent from normal tissues. Although LHRH and related peptides have been used to deliver cytotoxic drugs to LHRH-R overexpressing cancer cells, they have suffered from off-target delivery of the therapeutic agents to the liver and kidneys. To reduce such unwanted uptake by peptide scavenger receptors in the liver and kidneys, we have explored the use of a nonpeptidic LHRH-R antagonist (NBI42902) to construct an LHRH-R targeted tubulysin conjugate (BOEPL-L3-TubBH). In vitro studies with BOEPL-L3-TubBH demonstrate that the conjugate can kill LHRH-R expressing triple-negative breast cancer cells (MDA-MB-231 cells) with low nanomolar IC50. Related studies on tumor-bearing mice further reveal that the same conjugate can eradicate MDA-MB-231 solid tumors without any measurable side-effects, yielding mice that gain weight during therapy and show no evidence of tumor recurrence for at least 5 weeks after termination of treatment. That these complete responses are LHRH-R targeted was then established by showing that identical treatment of receptor-negative (SKOV3) tumors yields no antitumor response. Overall, these data provide a proof-of-concept that LHRH-R specific targeting of an extremely toxic drug like tubulysin B can treat LHRH-R positive tumors without causing significant toxicity to healthy cells.