To enhance PARPi sensitivity and overcome PARPi resistance, several feasible strategies should be considered and implemented in the future (Table
2): 1) PARPi-oncolytic herpes simplex viruses (oHSVs) combination; oHSVs, approved by FDA for recurrent melanoma, are genetically engineered to selectively kill cancer cells, due to their characteristics of amplifying and spreading within the tumor but not normal tissue. They are actively involved in manipulating DDR [
126]. Recently, MG18L, a newly identified activity of oHSV, was reported to proteasomally degrade RAD51 and sensitize glioblastoma stem cells (GSCs) to PARPi killing in synthetic lethal-like fashion in vivo and in vitro. The combination of olaparib with MG18L greatly increased survival in both PARPi-sensitive and -resistant GSC-derived tumors. The combination therapy not only overcomes PARPi resistance but also expands its use to tumors with HR-proficient. Most importantly, oHSVs only infect and kill tumor cells but not normal cells compared to conventional medicines, which means that they may have fewer side effects [
127]. Due to its broad anti-tumor efficacy in most solid tumors, this novel combination therapy should be applicable to other cancer stem cells and tumors; 2) PARPi-ionizing radiation (IR) combination; Nuclear localization is required for BRCA1 to participate in HR-mediated DNA repair [
128]. IR can initiate the export of BRCA1 from the nucleus to the cytoplasm, leading to increased sensitivity of PARPi in wild-type BRCA1 and HR-proficient tumor cells [
129,
130] However, because of the synthetic lethality of the combination therapy is p53-depend, it can only be used in wild-type p53 patients [
131]. Meanwhile, PARPi induce radiosensitization in vitro and in vivo models [
132]. What’s even more refreshing is HR restoration by 53BP1 pathway inactivation further increased radiosensitivity in preclinical model systems. It was showed that BRCA1-mutated tumors, which acquired drug resistance due to BRCA1-independent HR restoration, could be sensitized to radiotherapy [
133]. In addition to the preclinical results, clinical studies were also attempted to exploit the efficacy of PARPi-IR combination. A phase 1, open-label dose escalation study (NCT00649207) evaluating veliparib in combination with whole brain radiation therapy (WBRT) in patients with brain metastases were originated with Mehta and his colleagues [
134]. The preliminary efficacy results were better than predicted outcome based on the graded prognostic factors in the published nomogram. Based on encouraging safety and preliminary efficacy results, a randomized, controlled phase 2b study is ongoing. Other two phase 1 trials (NCT01264432, NCT01589419) indicated that the PARPi-IR combination treatment was well-tolerated and show good responses as well [
135,
136]. Undoubtedly, further evaluation of PARPi-IR combination treatments is currently underway in multiple phase 2 clinical trials in patients with NSCLC and breast cancer (NCT02412371, NCT01386385, NCT01618357). 3) PARPi-CDKs inhibitors; DNA end resection is depended on cyclin-dependent kinases (CDKs) activity. A number of studies indicated that CDKs played important roles in PARPi resistance [
36‐
41]. CDK inhibitor dinaciclib resensitized TBNC cells, which had acquired resistance to niraparib. In addition to TBNC cells, synthetic lethal strategy combining dinaciclib with niraparib was also highly efficacious in ovarian, prostate, pancreatic, colon, and lung cancer cells [
137]. Currently, CDK12 attracted more attentions in PARPi resistance, due to its inactivating somatic alterations were recurrently observed in various cancers. Numerous evidences proved that CDK12 mutation or deficiency lead to cancer cells sensitivity to PARPi [
37]. Furthermore, CDK12 inhibitors reversed de novo and acquired PARPi resistance in BRCA1-mutant breast cancer cells [
39]. 4) PARPi-immunotherapy; Jiao et al and her colleagues revealed that PARPi upregulated PD-L1 expression in breast cancer cell lines via inactivating GSK3β, which in return leading to attenuate anticancer immunity. Moreover, the combination of PARPi and anti-PD-L1 therapy showed better therapeutic efficacy than each alone [
138]. PARPi-mediated modulation of the immune response contributes to their therapeutic effects independently of BRCA1/2 mutations. Recently results suggested that PARPi promoted accumulation of cytosolic DNA fragments because of unresolved DNA lesions, which in turn activated the DNA-sensing cGAS-STING pathway and stimulated production of type I interferons to induce antitumor immunity independent of BRCAness [
139]. At present, several clinical trials (NCT02734004, NCT03824704 and NCT02849496) are ongoing. In this term, all these trails may be informative. 5) PARPi-epigenetic drugs; As previously mentioned, epigenetic modification was associated with PARPi sensitivity [
113,
117,
118]. Acetylation and deacetylation of histones is one of the most important mechanisms of posttranslational regulation of gene expression [
140]. So far, numerous studies have declared that treating with histone deacetylation inhibitors (HDACi) and PARPi exhibited synergy effects due to the induction of HDACi on HRD, which as a result sensitized cancer cells to PARPi [
141‐
144]. Several mechanisms have been observed. Firstly, it was reported that HDACi decreased the expression of DNA repair genes such as RAD51, CHK1, BRCA1 and RAD21 mediated through transcription factor E2F1 [
145]. Secondly, HDACi blocked the deacetylation and expression of HSP90, resulting in the degradation of its substrates BRCA1, Rad52, ATR and CHK1 [
146]. Finally, recent studies showed that acetylation blocked DNA damage-induced chromatin PARylation and HDACi treatment significantly increased the trapping of PARP1 at DSB sites in chromatin [
147,
148]. Additionally, low doses of DNA methyltransferase inhibitor (DNMTi) induced BRCAness phenotype through downregulating expression of key HR genes [
149]. The combination DNMTi and PARPi enhanced the cytotoxic effect by increasing the PARP “trapping” on DSB sites independent on BRCA mutations [
150,
151]. However, there is no clinical trial to evaluate its effect until now. 6) PARPi-other drugs; In addition to the above mentioned, PARPi was also suggested to combinate with HSP90 inhibitors, ATR/CHK1 inhibitors and WEE1 inhibitors [
152,
153]. BRCA1 function is reliant on HSP90. HSP90 inhibitor, 17-AAG, could induce HRD and increase Olaparib sensitivity of HR-proficient ovarian cancer cell lines [
154]. Treating PARPi-resistant cells with 7-dimethylaminoethylamino-17-demethoxygeldanamycin, a HSP90 inhibitor, reversed the resistance state by decreasing the quantity of BRCA1 protein [
92]. ATR/CHK1 and WEE1 have emerged as putative BRCAness factors that function in both checkpoint activation and in replication fork stability. ATR/CHK1 inhibitors and WEE1 inhibitors treatment were recently shown to reverse PARPi resistance in cancer cells [
152]. Currently, several trails to the safety and efficacy of these combination treatments in sporadic cancers are in progress (NCT03579316, NCT04197713, NCT02576444, NCT02511795, NCT04065269, NCT03787680, NCT03330847, NCT03878095, NCT03462342, NCT03428607, NCT03682289). In a word, the combination therapy to overcome PARPi resistance and enhance PARPi sensitivity is still in its infancy and has a long way to go. More and more studies are needed to investigate the feasibility in clinic.
Table 2
The feasible combination therapy to enhance PARPi sensitivity and overcome PARPi resistance
PARPi-oHSVs combination | No | | | | | |
PARPi-IR combination | Yes | NCT00649207 | I | Veliparib + WBRTa | Completed | Solid tumors with brain metastases |
PARPi-IR combination | Yes | NCT01264432 | I | Veliparib + IR | Completed | Peritoneal carcinomatosis; fallopian tube, ovarian and primary peritoneal cancers |
PARPi-IR combination | Yes | NCT01589419 | I | Veliparib + capecitabine + IR | Completed | Locally advanced rectal cancer |
PARPi-IR combination | Yes | NCT02412371 | I/II | Veliparib + Paclitaxel/Carboplatin + IR | Completed | Stage III NSCLCb |
PARPi-IR combination | Yes | NCT01386385 | I/II | Veliparib + Paclitaxel/Carboplatin + IR | Active, not recruiting | Stage III NSCLC |
PARPi-IR combination | Yes | NCT01618357 | I | Veliparib + IR | Recruiting | Breast cancer |
PARPi-CDKi combination | No | | | | | |
PARPi-immunotherapy | Yes | NCT02734004 | I/II | Olaparib + MED14736 | Active, not recruiting | Ovarian, breast, SCLC cand gastric cancers |
PARPi-immunotherapy | Yes | NCT03824704 | II | Rucaparib + Nivolumab | Active, not recruiting | Epithelia ovarian cancer, fallopian tube cancer, primary peritoneal cancer, HGSCd and endometrioid adenocarcinoma |
PARPi-immunotherapy | Yes | NCT02849496 | II | Olaparib + Atezolizumab | Recruiting | Locally advanced unresectable; metastatic non-HER2-positive breast cancer |
PARPi- epigenetic drugs | No | | | | | |
PARPi- HSP90 inhibitors | No | | | | | |
PARPi-WEE1 inhibitors | Yes | NCT03579316 | II | Olaparib + AZD1775 | Recruiting | Recurrent fallopian tube, ovarian and primary peritoneal cancers |
PARPi-WEE1 inhibitors | Yes | NCT04197713 | I | Olaparib + AZD1775 | Not yet recruiting | Advanced solid tumors with selected mutations and PARP Resistance |
PARPi-WEE1 inhibitors | Yes | NCT02576444 | II | Olaparib + AZD1775 | Active, not recruiting | Tumors harboring either TP53 or KRAS mutations or mutations in KRAS and TP53 |
PARPi-WEE1 inhibitors | Yes | NCT02511795 | I | Olaparib + AZD1775 | Completed | Refractory solid tumors; Relapsed SCLC |
PARPi-ATR inhibitors | Yes | NCT02576444 | II | Olaparib + AZD6738 | Active, not recruiting | Tumors harboring mutations leading to dysregulation of the PI3K/AKT pathway |
PARPi-ATR inhibitors | Yes | NCT04065269 | II | Olaparib + AZD6738 | Recruiting | Gynaecological cancers |
PARPi-ATR inhibitors | Yes | NCT03787680 | II | Olaparib + AZD6738 | Recruiting | Prostate cancer |
PARPi-WEE1/ATR inhibitors | Yes | NCT03330847 | II | Olaparib + AZD6738/ AZD1775 | Recruiting | Metastatic triple negative breast cancer |
PARPi-ATR inhibitors | Yes | NCT03878095 | II | Olaparib + AZD6738 | Recruiting | IDH1 and IDH2 mutant tumors |
PARPi-ATR inhibitors | Yes | NCT03462342 | II | Olaparib + AZD6738 | Recruiting | HGSC |
PARPi-ATR inhibitors | Yes | NCT03428607 | II | Olaparib + AZD6738 | Active, not recruiting | SCLC |
PARPi-ATR inhibitors | Yes | NCT03682289 | II | Olaparib + AZD6738 | Recruiting | Clear cell renal cell cancer; Metastatic renal cell cancer; Metastatic urothelial cancer; Metastatic pancreatic cancer; Locally advanced pancreatic cancer |