PARP Inhibitors: The Cornerstone of DNA Repair–Targeted Therapies
PARP Inhibitors: The Cornerstone of DNA Repair–Targeted Therapies
The activity and therapeutic licensing of poly(ADP-ribose) polymerase (PARP) inhibitors is the culmination of 50 years of research. However, the biology, mechanisms of action, adequate treatment combinations, and targeted populations for these agents need to be explored further. PARP activity is essential for the repair of single-strand DNA breaks via the base excision repair pathway. This pathway is the default repair pathway in cells with deficient high-fidelity double-strand break homologous recombination (HR) repair, such as occurs with loss of BRCA1 or BRCA2 function. Therefore, inhibition of PARP function results in cell death in HR-deficient tumors, and sensitizes tumor cells to cytotoxic agents that induce DNA damage. Applications of PARP inhibition are now being expanded beyond tumors with HR deficiency—to HR-competent tumors in which HR has been synthetically impaired through use of other agents given in combination with PARP inhibitors, or resulting from PARP inhibition in the setting of BRCA1 or BRCA2 loss.
The discovery of the first poly(ADP-ribose) polymerase (PARP) was made over 50 years ago in Paul Mandel’s laboratory, with the observation of the synthesis of a new polyadenylic acid after nicotinamide mononucleotide had been added to rat liver extracts. By 1980 it was known that this nuclear enzyme was activated by DNA damage and played an essential role in the repair of DNA.[2-4] There are now 17 known members of the PARP nuclear superfamily, with PARP1 and PARP2 as the two that are predominantly involved in DNA repair.
It was discovered that in cancer cells in which repair of DNA is already impaired, inhibition of PARP, by increasing genomic instability, can result in the death of the tumor cells. The antitumor effects of PARP inhibition were first demonstrated in ovarian cancer cells.[3,4,7,8] Olaparib was the first PARP inhibitor to be approved by the US Food and Drug Administration (FDA); it was granted accelerated approval for use in BRCA1/2 mutation carriers in the fourth or greater treatment line, based on clinical data reporting antitumor activity that resulted in significant clinical benefit in high-grade serous ovarian cancer (HGSOC) with germline mutations in the BRCA1 or BRCA2 gene. Olaparib is approved in Europe for women with either germline or somatic deleterious mutations for maintenance of chemotherapy response to second- or later-line platinum-based therapy, after completion of the last platinum regimen. Olaparib has also been approved by Health Canada.
Ongoing phase II and III clinical trials are testing the efficacy of currently available PARP inhibitors—olaparib, niraparib, rucaparib, veliparib, and talazoparib—that have also shown antitumor efficacy and tolerability alone or when adding a synergistic effect in combination with other agents (Table 1). Rucaparib just received FDA approval in December 2016 for use in women with HGSOC in the third or greater line, especially patients with homologous recombination (HR) dysfunction as shown by an HR dysfunction companion diagnostic. Niraparib activity in maintenance of chemotherapy response in patients with recurrent ovarian cancer was reported in November 2016 and is awaiting review by the FDA and the European Medicines Agency (EMA).
DNA Repair and the Role of PARP
Two major forms of DNA damage can occur: single-strand breaks (SSBs) and double-strand breaks (DSBs). Thus, there are two repair pathway categories: SSB-targeted and DSB-targeted repair pathways. Base excision repair (BER) is one of several pathways involved in the repair of selected types of DNA SSBs. PARP1, via a process known as PARylation (poly[ADP-ribose] polymerization), plays an important role in the BER of DNA SSBs. In the nucleus, PARP1 senses SSB DNA injury and recruits DNA repair complexes to the site of SSBs (Figure 1).
If PARylation is inhibited and BER is impaired, unrepaired or misrepaired SSBs accumulate, and at replication forks they degenerate to become DSBs. DSBs are repaired predominantly by the high-fidelity HR repair pathway that is active in the G2 phase of the cell cycle. The low-fidelity nonhomologous end-joining (NHEJ) program, which functions predominantly in the G1/S transition and S phase, is a poor-quality DSB repair backup; this form of repair religates DNA strands indiscriminately, introducing new DNA errors that cumulatively can result in cell death. (Figure 2).
BRCA1 and BRCA2 are tumor suppressor genes known for the association of their deleterious germline mutations with familial, high-penetrance breast and ovarian cancers. Both BRCA1 and BRCA2 are involved in HR repair of DSBs and help maintain genomic stability. Cells with homozygous loss of normal BRCA1/2 function have defective HR and must rely more on the low-fidelity NHEJ program to repair DNA DSBs. Thus, cells with deleterious mutations in BRCA1 or BRCA2 are more dependent on BER and PARP1 for rescue and maintenance of genomic stability—and consequently more susceptible to impairment of the BER pathway. In such cells, PARP inhibition results in further genomic instability and cell death.
In addition to tumors with deleterious germline BRCA1 (gBRCA1) or gBRCA2 mutations, other tumors in which HR is dysfunctional, such as those in which HR dysfunction results from genomic events besides germline mutations—eg, homozygous loss by somatic mutation, silencing by methylation, or loss of other HR proteins, such as RAD51c—are also more susceptible to PARP inhibition. In such settings, a program of clinical “synthetic lethality” is activated. Synthetic lethality exists when two conditions that independently would not cause cell death are applied in combination and together cause lethal injury.[12,13] Cancer cells already have impaired DNA repair function and a higher burden of genomic injury than normal cells; in tumors in which there has also been genomic loss of DNA repair function or other impairment of HR, synthetic lethality, as induced by PARP inhibition, may be used to target tumor tissue selectively, with decreased toxicity in normal cells that have intact repair pathways and thus are more able to survive an onslaught of DNA repair inhibitors.[14,15]
Development of PARP Inhibitors as Viable Anticancer Agents
The finding that PARP inhibitors were at least additive with platinums and other agents and resulted in clinical synthetic lethality when applied to HR-deficient cells led to the development of several NAD-mimetic PARP inhibitors. It is unclear which of these agents’ many mechanisms of action are primary to their clinical activity. PARP inhibitors block the PARylation that normally occurs in response to DNA damage. All PARP inhibitors have the capacity to trap the PARP enzyme on injured DNA, preventing binding of incoming repair proteins. Both the binding and dissociation affinities are critical, as the longer the PARP enzyme is trapped on DNA (a result of higher affinity), the greater the benefit observed in preclinical models. The other major role of PARP inhibition is regulation of DNA-dependent protein kinase, catalytic subunit (DNA-PKcs), the rate-limiting enzyme involved in NHEJ. PARP1 keeps DNA-PKcs in an inactive mode, inhibiting NHEJ activity. Upon loss of PARP1, DNA-PKcs is phosphorylated and activated, thereby promoting the dysfunctional and injurious DSB repair performed by the NHEJ program.