Cancer Research: A Very Long-Acting PARP Inhibitor Suppresses Cancer Cell Growth in DNA Repair-Deficient Tumor Models (Houghton, Kurmasheva)

Shaun D. Fontaine, Gary W. Ashley, Peter J. Houghton, Raushan T. Kurmasheva, Morgan Diolaiti, Alan Ashworth, Cody J. Peer, Ryan Nguyen, William D. Figg Sr., Denis R. Beckford-Vera, and Daniel V. Santi


PARP inhibitors are approved for the treatment of cancers with BRCA1 or BRCA2 defects. In this study, we prepared and characterized a very long-acting PARP inhibitor. Synthesis of a macromolecular prodrug of talazoparib (TLZ) was achieved by covalent conjugation to a PEG40kDa carrier via a β-eliminative releasable linker. A single injection of the PEG∼TLZ conjugate was as effective as ∼30 daily oral doses of TLZ in growth suppression of homologous recombination-defective tumors in mouse xenografts. These included the KT-10 Wilms’ tumor with a PALB2 mutation, the BRCA1-deficient MX-1 triple-negative breast cancer, and the BRCA2-deficient DLD-1 colon cancer; the prodrug did not inhibit an isogenic DLD-1 tumor with wild-type BRCA2. Although the half-life of PEG∼TLZ and released TLZ in the mouse was only ∼1 day, the exposure of released TLZ from a single safe, effective dose of the prodrug exceeded that of oral TLZ given daily over one month. μPET/CT imaging showed high uptake and prolonged retention of an 89Zr-labeled surrogate of PEG∼TLZ in the MX-1 BRCA1-deficient tumor. These data suggest that the long-lasting antitumor effect of the prodrug is due to a combination of its long t1/2, the high exposure of TLZ released from the prodrug, increased tumor sensitivity upon continued exposure, and tumor accumulation. Using pharmacokinetic parameters of TLZ in humans, we designed a long-acting PEG∼TLZ for humans that may be superior in efficacy to daily oral TLZ and would be useful for the treatment of PARP inhibitor-sensitive cancers in which oral medications are not tolerated.

Significance: These findings demonstrate that a single injection of a long-acting prodrug of the PARP inhibitor talazoparib in murine xenografts provides tumor suppression equivalent to a month of daily dosing of talazoparib.

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