HRD-Directed Therapeutics Hold Potential in Breast Cancer, But Challenges Remain

OncologyLiveVol. 24/No. 6
Volume 24
Issue 6

Roadblocks such as PARP inhibitor resistance have resulted in an unmet need and uncertain optimal identification concerning patients who would elicit the most benefit from homologous recombination deficiency–directed therapies.

Chris Lord, MD

Chris Lord, MD

Tumors exhibiting homologous recombination deficiency (HRD) have traditionally been treated with targeted therapies against BRCA1/2; however, roadblocks such as PARP inhibitor resistance have resulted in an unmet need and uncertain optimal identification concerning patients who would elicit the most benefit from HRD-directed therapies. Data on emerging therapies and early investigative efforts to uncover the root of resistance were presented during the 2022 San Antonio Breast Cancer Symposium (SABCS). Findings from these studies showed that a deeper understanding of genetics and a sharper focus on PARP1 signal actionable paths forward, but further work remains to more effectively target genes involved with HRD.1-3

HRD is defined as the inability of a cell to effectively repair DNA double-strand breaks using the homologous recombination repair (HRR) pathway. Deficiencies in the HRR pathway have been identified in multiple tumor types, including breast, ovarian, and prostate cancers. Affected HRR genes associated with the HRD phenotype include BRCA1/2, ATMPALB2, and RAD51.4

In breast cancer, mutations in BRCA1/2 have been strongly linked with HRD, and the development of HRD can contribute to the growth of breast cancer. However, there are patients with BRCA1/2 wild-type breast cancer that also displays HRD. Chemotherapy-based therapeutic strategies and PARP inhibitors have shown some efficacy in targeting the HRD, with the latter gaining significant popularity in the space because of the lack of toxicity that typically accompanies treatment with chemotherapy.5

Two PARP inhibitors are approved by the FDA for patients with breast cancer: talazoparib (Talzenna) and olaparib (Lynparza). On October 16, 2018, the FDA approved talazoparib for patients with deleterious or suspected deleterious germline BRCA-mutated, HER2-negative locally advanced or metastatic breast cancer. On March 11, 2022, the agency approved olaparib for the adjuvant treatment of adult patients with deleterious or suspected deleterious germline BRCA-mutated, HER2-negative, high-risk earlystage breast cancer who have been treated with neoadjuvant or adjuvant chemotherapy.6,7

Several mechanisms of resistance to PARP inhibition can develop, such as the restoration of homologous recombination proficiency, the transition to alternate DNA repair mechanisms, decreased PARP expression and binding, and secondary mutations in homologous recombination–related genes. All these mechanisms can greatly reduce or even eliminate the therapeutic impact of PARP inhibitors.8

Identifying and Overcoming the Mechanisms of PARP Inhibitor Resistance

The homologous recombination and nonhomologous end joining (NHEJ) DNA repair pathways are 2 areas in which double strand breaks can be addressed; the former is associated with PARP inhibition.1 Repairing double-strand breaks is largely based on the functionality of the DNA repair proteins BRCA1 and 53BP1.1,9 If the doublestrand break is not resected, 53BP1 is functional and is repaired by NHEJ. If BRCA1 is functional, the double-strand break is resected, which has been identified as a prelude to homologous recombination.

However, cells may also use another pathway, θ-mediated end joining, also known as microhomology-mediated end joining.1,9,10 During the 2022 SABCS, Chris Lord, MD, director of the Breast Cancer Now Toby Robins Research Centre at the Institute of Cancer Research in London, England, cited how identifying resistance mechanisms, such as the loss of 53BP1, is the first step to developing ways to target the restoration of homologous recombination. He noted that polymerase theta (Polθ) represents one such route, using the agents ART558 and novobiocin, which are inhibitors of the polymerase activity not the ATP activity of Polθ.1,9-11

The agent elicited synthetic lethality of BRCA1 activity and synergized with PARP inhibitors, Lord explained.1 Additionally, it allowed for the targeting of BRCA1-mutant PARP inhibitor–resistant tumor cells that lost 53BP1 or the Shieldin complex (SHD3, SHD2, SHD1). Importantly, Lord noted, from a clinical perspective, this does not work in the setting of a reversion mutation if there is a preexisting reversion mutation as the mutation; this is because the mutation, in addition to causing resistance to a PARP inhibitor, also causes resistance to a Polθ inhibitor.1

Overexpression of Polθ in some types of breast and ovarian cancers also displaying HRD makes it an intriguing treatment target as inhibition. ART558 is a potent and specific small molecule inhibitor of Polθ that has displayed synthetic lethality and a combinatorial effect with olaparib in isogenic models of BRCA1 deficiency and in tumor cell lines with endogenous pathogenic BRCA1 mutations.10

Another Polθ inhibitor, the first-in-class antibiotic novobiocin, has shown selective killing of HRD tumor cells both in vitro and in vivo. The agent directly binds to the Polθ ATPase domain, inhibiting its ATPase activity, and ultimately phenocopies Polθ depletion.11

After identifying novobiocin as a Polθ inhibitor via a small molecule screen, investigators evaluated its specificity and found that novobiocin-conjugated beads pulled down the purified ATPase domain of POLθ, without affecting purified SMARCAL1, CHD1, BLM, or RAD51. Additionally, they found that in a genetically engineered mouse model of triple-negative breast cancer (TNBC), novobiocin-treated tumors were significantly smaller than vehicle-treated tumors following a week of treatment. Novobiocin was shown to prolong the median overall survival of the tumor-bearing mice vs vehicle at 29 days vs 10 days, respectively. The safety of the agent had already been validated in previous anticancer clinical trials.11

Study authors also noted that novobiocin showed synergy with PARP inhibitors. Notably, in mouse models with HRD DF83 ovarian cancer that received novobiocin in combination with olaparib, complete tumor regression was reported with few tumor cells remaining detectable with bioluminescence imaging. Additionally, in a PARP inhibitor-resistant mouse model, treatment with novobiocin monotherapy substantially reduced tumor growth, with no response being observed with olaparib monotherapy. The combination of the 2 agents was even more effective with tumor regression being reported in the first 2 weeks.11

Investigators concluded that Polθ expression is a predictive biomarker for novobiocin sensitivity and that many tumors that have developed resistance to PARP inhibition are likely to remain sensitive to novobiocin, depending on their mechanism of resistance.11

Novel HRD Therapies Showing Promise

In addition to overcoming PARP resistance, novel therapies are being developed and that have shown promise in the treatment of patients with HRD breast cancer. In another presentation at the 2022 SABCS, Andrew Tutt, MB ChB, PhD, MRCP, FRCR, also of the Institute of Cancer Research, reviewed data for ATR inhibitors and PARP inhibitors that are selective for PARP1.3

Following preclinical study showing that ATR inhibition enhanced efficacy in models with HRD and/or PARP-inhibitor resistance, investigators initiated the phase 2 VIOLETTE study (NCT03330847). VIOLETTE enrolled patients with TNBC and randomly assigned them 1:1:1 to receive either olaparib 300 mg twice daily for a 28-day cycle (n = 150); olaparib 300 mg twice daily for a 28-day cycle plus the ATR inhibitor ceralasertib at a dose of 160 mg daily on days 1 through 7 (n = 150); or olaparib 200 mg daily via a continuous 21-day cycle in combination with the WEE1 inhibitor adavosertib. Patients needed to have previously undergone 1 or 2 lines of metastatic breast cancerdirected chemotherapy to be eligible for the study.12

Figure. Pathways Implicated in PARP Inhibitor Resistance9

Figure. Pathways Implicated in PARP Inhibitor Resistance9

The primary end point was progression-free survival (PFS). Key secondary end points included objective response rate (ORR), duration of response (DOR), change in target lesion size, safety, and tolerability.

Findings from the trial showed that the median PFS was 5.3 months (90% CI, 3.7-5.5) in the ceralasertib arm vs 3.6 months (90% CI, 2.9-5.4) in the monotherapy arm (HR, 0.79; 90% CI, 0.59-1.04; P = .1822). The ORR was 28.6% vs 21.1%, respectively (OR, 1.50; 90% CI, 0.90-2.51; P = .1932).12

However, patients with non–HRR-mutant disease who received monotherapy (n = 51) experienced an ORR of only 3.9% compared with 15.4% among 52 patients in this subgroup who received the ceralasertib combination (OR, 4.45; 1.30-21.20; P = .0425). Median DOR was also significantly improved in this subgroup, at 11.4 months (90% CI, 7.1-15.7) vs 24.1 months (90% CI, 15.1-24.1), respectively. Tutt mentioned that these findings point to patients with non–HRR-mutant disease as a possible signal subgroup for this treatment regimen.

Tutt noted that early stoppage of the trial limited interpretation of the results. Additionally, the dose of ceralasertib used in the study was lower than that being examined in monotherapy studies. He also emphasized that VIOLETTE was primarily focused on PARP inhibitor–naïve patients to test ATR dependence and the efficacy of ceralasertib. Study authors also noted that further investigation was warranted for ceralasertib plus olaparib among patients who relapse after or while receiving PARP inhibitors.12

PARP1 inhibition is another novel treatment approach that is piquing the interest of clinicians who are treating patients with HRD-positive tumors. Approved PARP inhibitors trap both PARP1 and PARP2. However, PARP2 has been shown to be more important for bone marrow function than PARP1. The next generation of PARP1 inhibitors must be able to reduce some of the toxicities of standard-of-care PARP inhibitor monotherapies without significantly reducing efficacy and enable tolerable combination regimen options with chemotherapy or ATR inhibitors.3

In the ongoing phase 1/2a PETRA trial (NCT04644068) trial, investigators are examining the safety and preliminary efficacy of AZD5305 in patients with advanced breast, ovarian, prostate, or pancreatic cancer harboring BRCA1/2, PALB2 or RAD51C/D mutations. AZD5305 is a potent, highly selective PARP1 inhibitor that is 500-fold more selective for PARP1 than PARP2 and is also very selective for HRD.3 The agent exhibited superior preclinical tolerability, target engagement, and efficacy compared with traditional first-generation PARP inhibitors that target both PARP1 and PARP2.13

Prior treatment with a PARP inhibitor and/or chemotherapy was permitted as part of the trial. The primary end point was safety. Secondary end points included pharmacokinetics, pharmacodynamics, and ORR.

At the November 17, 2021, data cutoff patients were treated with AZD5305 at a dose ranging from 10 to 90 mg daily (n = 46). Responses were observed starting at the lowest dose levels; efficacy-evaluable patients (n = 25) experienced an ORR of 28%, including responses in patients who were PARP inhibitor or platinum resistant. Thirteen of 22 RECIST-measurable patients had stable disease or a partial response up to 51 weeks. The agent was also found to be well tolerated across dose levels and no dose-limiting toxicities were reported.


  1. Lord C. Mechanisms of sensitivity and resistance to HRD targeting agents in breast cancer – PARPi and beyond. Abstract presented at: 2022 San Antonio Breast Cancer Symposium; December 6-10, 2022; San Antonio, TX.
  2. d’Andrea A. Pol-theta inhibitors in HRD tumors. Abstract presented at: 2022 San Antonio Breast Cancer Symposium; December 6-10, 2022; San Antonio, TX.
  3. Tutt A. ATR inhibitors and PARP1 selective PARP inhibitors. Abstract presented at: 2022 San Antonio Breast Cancer Symposium; December 6-10, 2022; San Antonio, TX.
  4. Stewart MD, Merino Vega D, Arend RC, et al. Homologous recombination deficiency: concepts, definitions, and assays. Oncologist. 2022;27(3):167-174. doi:10.1093/oncolo/oyab053
  5. denBrok WD, Schrader KA, Sun S, et al. Homologous recombination deficiency in breast cancer: a clinical review. JCO Precis Oncol. 2017;1:1-13. doi:10.1200/PO.16.00031
  6. FDA approves talazoparib for gBRCAm HER2-negative locally advanced or metastatic breast cancer. FDA. Updated December 14, 2018. Accessed February 22, 2023.
  7. FDA approves olaparib for adjuvant treatment of high-risk early breast cancer. FDA. March 11, 2022. Accessed February 22, 2023.
  8. Mekonnen N, Yang H, Shin YK. Homologous recombination deficiency in ovarian, breast, colorectal, pancreatic, non–small cell lung and prostate cancers, and the mechanisms of resistance to PARP inhibitors. Front Oncol. 2022;12:880643. doi:10.3389/fonc.2022.880643
  9. Li H, Liu ZY, Wu N, Chen YC, Cheng Q, Wang J. PARP inhibitor resistance: the underlying mechanisms and clinical implications. Mol Cancer. 2020;19(1):107. doi:10.1186/s12943-020-01227-0
  10. Zatreanu D, Robinson HMR, Alkhatib O, et al. Polθ inhibitors elicit BRCA-gene synthetic lethality and target PARP inhibitor resistance. Nat Commun. 2021;12(1):3636.doi:10.1038/s41467-021-23463-8
  11. Zhou J, Gelot C, Pantelidou C, et al. A first-in-class polymerase theta inhibitor selectively targets homologous-recombination-deficient tumors. Nat Cancer. 2021;2(6):598-610. doi:10.1038/s43018-021-00203-x
  12. Tutt A, Nowecki Z, Szoszkiewicz R, et al. VIOLETTE: randomisedphase 2 study of olaparib (ola) +ceralasertib (cer) oradavosertib (ada) vs ola alone in patients (pts) with metastatic triple-negative breast cancer (mTNBC). Ann Oncol. 2022;33(suppl 3):S194-S195. doi:10.1016/j.annonc.2022.03.180
  13. Yap TA, Im SA, Schram AM, et al. PETRA: first in class, first in human trial of the next generation PARP1-selective inhibitor AZD5305 in patients (pts) with BRCA1/2, PALB2 or RAD51C/D mutations. Cancer Res. 2022;82(suppl 12):CT007. doi:10.1158/1538-7445.AM2022-CT007
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