Targeting Polθ Helicase with RP-3467: Preclinical Validation and Clinical Response in a gBRCA1 ovarian cancer patient with an acquired PARPi resistance mutation in TP53BP1
- 1Memorial Sloan Kettering Cancer Center
Abstract
Homologous recombination (HR) deficiency underlies a range of cancers, and PARP inhibitors (PARPi) targeting this vulnerability have demonstrated clinical efficacy in BRCA1/2-mutant tumors. However, resistance mechanisms and tolerability issues limit their long-term clinical effectiveness. Here, we characterize RP-3467, a novel, selective small-molecule inhibitor targeting the ATPase domain of DNA polymerase theta (Pol{theta}), the key mediator of the error-prone microhomology-mediated end joining (MMEJ) pathway. We demonstrate that RP-3467 potently suppresses Pol{theta} enzymatic activity, abrogates MMEJ, and disrupts mitotic DNA repair in HR-deficient (HRD) models, inducing synthetic lethality. In preclinical studies, we show that RP-3467 synergizes strongly with PARPi (olaparib and rucaparib), achieving sustained tumor regression across multiple xenograft and patient-derived xenograft (PDX) models without added systemic toxicity. Mechanistically, we find that RP-3467 increases mitotic DNA damage in vivo, as evidenced by elevated micronuclei and CIP2A foci. RP-3467 was evaluated in a phase 1 clinical trial (NCT06560632) and has shown preliminary signs of clinical activity. A heavily pretreated germline BRCA1-mutant ovarian cancer patient achieved a confirmed partial response to RP-3467 plus olaparib (RECIST v1.1), with retrospective profiling revealing a TP53BP1 loss-of-function mutation previously linked to both PARPi resistance and POLQ dependency. Although broader clinical validation is needed, these early findings suggest that Pol{theta} inhibition has the potential to address PARPi resistance and offer therapeutic benefit in HRD cancers.
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BibTeX
@misc{sfeir2026targeting,
title = {{Targeting Polθ Helicase with RP-3467: Preclinical Validation and Clinical Response in a gBRCA1 ovarian cancer patient with an acquired PARPi resistance mutation in TP53BP1}},
author = {A. Sfeir and E. Rosen and M.-C. Mathieu and G. B. Ferraro and P. Mochirian and C. Godbout and H. Poirier and S. Fournier and D. A. Henry and H. Kim and R. Papp and S. Y. Yin and M.-E. Leclaire and R. Houle and P. Nejad and J. D. Schonhoft and K. Zahn and W. C. Black and M. Gallant and I. Kim and D. Ulanet and P. Basciano and A. Veloso and V. Rimkunas and T. A. Yap and R. Guo and N. Mai and M. Koehler and M. Zinda and A. Roulston and M. Zimmermann and S. J. Morris},
howpublished = {medRxiv},
year = {2026},
month = sep,
doi = {10.64898/2026.09.16.26363059},
url = {https://www.medrxiv.org/content/10.64898/2026.09.16.26363059v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Targeting Polθ Helicase with RP-3467: Preclinical Validation and Clinical Response in a gBRCA1 ovarian cancer patient with an acquired PARPi resistance mutation in TP53BP1
AU - A. Sfeir
AU - E. Rosen
AU - M.-C. Mathieu
AU - G. B. Ferraro
AU - P. Mochirian
AU - C. Godbout
AU - H. Poirier
AU - S. Fournier
AU - D. A. Henry
AU - H. Kim
AU - R. Papp
AU - S. Y. Yin
AU - M.-E. Leclaire
AU - R. Houle
AU - P. Nejad
AU - J. D. Schonhoft
AU - K. Zahn
AU - W. C. Black
AU - M. Gallant
AU - I. Kim
AU - D. Ulanet
AU - P. Basciano
AU - A. Veloso
AU - V. Rimkunas
AU - T. A. Yap
AU - R. Guo
AU - N. Mai
AU - M. Koehler
AU - M. Zinda
AU - A. Roulston
AU - M. Zimmermann
AU - S. J. Morris
PY - 2026
DA - 2026/09/16
PB - medRxiv
DO - 10.64898/2026.09.16.26363059
UR - https://www.medrxiv.org/content/10.64898/2026.09.16.26363059v1
AB - Homologous recombination (HR) deficiency underlies a range of cancers, and PARP inhibitors (PARPi) targeting this vulnerability have demonstrated clinical efficacy in BRCA1/2-mutant tumors. However, resistance mechanisms and tolerability issues limit their long-term clinical effectiveness. Here, we characterize RP-3467, a novel, selective small-molecule inhibitor targeting the ATPase domain of DNA polymerase theta (Pol{theta}), the key mediator of the error-prone microhomology-mediated end joining (MMEJ) pathway. We demonstrate that RP-3467 potently suppresses Pol{theta} enzymatic activity, abrogates MMEJ, and disrupts mitotic DNA repair in HR-deficient (HRD) models, inducing synthetic lethality. In preclinical studies, we show that RP-3467 synergizes strongly with PARPi (olaparib and rucaparib), achieving sustained tumor regression across multiple xenograft and patient-derived xenograft (PDX) models without added systemic toxicity. Mechanistically, we find that RP-3467 increases mitotic DNA damage in vivo, as evidenced by elevated micronuclei and CIP2A foci. RP-3467 was evaluated in a phase 1 clinical trial (NCT06560632) and has shown preliminary signs of clinical activity. A heavily pretreated germline BRCA1-mutant ovarian cancer patient achieved a confirmed partial response to RP-3467 plus olaparib (RECIST v1.1), with retrospective profiling revealing a TP53BP1 loss-of-function mutation previously linked to both PARPi resistance and POLQ dependency. Although broader clinical validation is needed, these early findings suggest that Pol{theta} inhibition has the potential to address PARPi resistance and offer therapeutic benefit in HRD cancers.
ER -