PARP-1 inhibitors for anticancer therapy: where are we?Oqab AloqabC2007828AbstractPoly ADP-ribose polymerase-1 (PARP1) has recently emerged as a promising therapeutic target for the treatment of certain cancers, such as ovarian and breast cancer. Several compounds have been developed as PARP inhibitors, with some having gained approval and others still being tested in clinical trials. However, resistance to some of these drugs has been reported, which has led to many studies investigating strategies to overcome some of these resistance mechanisms and to develop new compounds.IntroductionIn both malignant and non-transformed human cells, poly ADP-ribose polymerase-1 (PARP1) is responsible for nearly 90% of all ADP-ribosyl transferase activity (Schiewer, 2014). The PARP1 enzyme plays an important role in DNA repair, which means it could also assist in repairing the damaged DNA in cancer cells. In recent years, PARP1 has become a significant therapeutic target in the treatment of certain cancers, such as ovarian and breast cancer. Moreover, some recent studies have found promising results against different cancer cells, such as hepatic carcinoma, as a result of PARP1 inhibition. Subsequently, many publications have been published with the aim of discovering new classes of anticancer drugs that can act by inhibiting PARP1. Furthermore, the Food and Drug Administration (FDA) has approved some of these drugs, which belong to a new group of anticancer drugs called PARP1 inhibitors. In addition, some other drugs are still undergoing pre-clinical trials and need to be further developed and enhanced.However, as a new class of anticancer drugs, it is natural to expect cancers to develop resistance against these drugs. Some studies have reported different mechanisms of resistance that have been shown clinically and are usually due to mutations in the cancer cells or overexpression of genes. These mechanisms, which have been reported both pre-clinically and in the clinic, are not fully understood. The problem of drug resistance needs to be overcome either by understanding its underlying mechanisms and then developing new compounds that prevent resistance from developing or by finding novel compounds that can inhibit the enzyme in a different way. These would be considered as solutions for the limitations that the currently approved drugs face, and may also open up a wider field of research in targeting PARP1. In this essay, I explore how PARP1 inhibitors have become a new class of drugs in anticancer therapy with regard to the basis of PARP1 as a drug target, The development of resistance to PARP inhibitors and the future of PARP1 inhibitors.PARP1 as a drug targetIn recent years, PARP1 has become a significant therapeutic drug target in the treatment of cancer for several significant reasons. First, it inhibits the repair pathway for cancer-causing mutations. Cancer cells can repair their DNA damage using PARP1; therefore, inhibition of this pathway impairs the ability of cancer cells to repair DNA damage, which in turn results in cell death. According to Christopher and Alan (2008), PARP inhibitors could potentially target homologous recombination (HR) deficiency, which is related to the increased susceptibility of several types of cancer and builds on a large body of research on PARP enzymes and the development of PARP inhibitors. The inhibition of PARP1 enzymatic activity causes an accumulation of single-strand breaks that are converted to double-strand breaks but cannot be restored by HR in cancer cells with breast cancer susceptibility protein (BRCA)1 or BRCA2 loss of function; these cells are deficient in their ability to repair DNA via HR (Konstantin et al., 2011). Moreover, PARP enzymes use nicotinamide adenine dinucleotide (NAD)+ as a substrate to catalyse the polymerisation of ADP-ribose moieties onto target proteins, releasing nicotinamide in the process. Designing new compounds that mimic nicotinamide would prevent PARP from repairing the damaged DNA. For example, 3-aminobenzamide is a nicotinamide analogue that has shown inhibitory effects by acting as a competitive inhibitor that can mimic the substrate and prevent it from binding to the protein. As a result, PARP can no longer bind to natural nicotinamide, which leads to a cessation in DNA repair (Christopher and Alan, 2008). After extensive research efforts in developing new PARP1 inhibitors, olaparib (Figure 1) became the first PARP1 inhibitor to be approved in the USA and the EU for the treatment of ovarian cancer associated with BRCA mutations (Emma, 2015). Figure 1. The structure of olaparibMoreover, the FDA has approved two other PARP inhibitors, rucaparib and niraparib, that can be used in the treatment of ovarian cancer. The approval of new drugs makes it easier to improve pharmacological properties such as selectivity, potency, stability, permeability and toxicity.Another important reason for considering PARP as a drug target is that many anticancer drugs, such as alkylating agents, work by damaging DNA. However, PARP proteins repair DNA, enabling cells to survive. PARP inhibition has advantages in terms of inducing DNA damage. Dragony (2012) argues that PARP1 can increase sensitivity to various alkylating agents, meaning that PARP inhibition may be used in conjunction with monofunctional alkylating agents to improve their efficacy. Thus, the combination of PARP1 inhibitors with DNA-damaging drugs may be more effective in preventing the PARP protein from repairing damaged DNA. According to Patrick (2019), in phase II of the BROCADE-2 trial, veliparib (a PARP1 inhibitor), when given in combination with carboplatin/paclitaxel chemotherapy for BRCA1/ 2-mutant metastatic breast cancer, showed some promising results, with an overall response rate (ORR) of 77.8%. Furthermore, Muria et al. (2014) noted that a combination of the alkylating agent temozolomide and the PARP1 inhibitor olaparib was more potent than when each drug was administered alone, and this was due to PARP1 being unable to repair the DNA damage induced by temezolomide. For the above reasons, PARP1 inhibition has led to improvements in the possibility of treating several cancer types, such as breast and ovarian cancer.The development of resistance to PARP inhibitorsIt is important to note that tumour cells have been demonstrated to exhibit resistance against PARP inhibitors. One of the largest challenges that scientists face is drug resistance.Tumour cells, such as those with BRCA1/2 mutations, are often sensitive to inhibitors that have been used as anticancer drugs; therefore, resistance against PARP inhibitors is common and develops through several mechanisms, as discussed below.One of the most common mechanisms of resistance against PARP inhibitors is somatic reversion or restoration of the open reading frame (ORF). In this mechanism, the tumour cells may be able to restore HR repair by inducing somatic reversion in a mutated BRCA1/2 gene; alternatively, the tumour cell may develop a new mechanism to protect its replication (Alan, 2018). As a result, the inherited mutation is reverted genetically, which also restores expression of the full-length wild-type protein or a functional protein that is nearly full-length.Another resistance mechanism that has been shown clinically is the mutation of PARP1-binding zinc fingers. Stephen et al. (2018) states that PARP1 inhibitor resistance is caused by mutations both within and outside the PARP1 DNA-binding zinc finger domains, as well as a PARP1 mutation discovered in a clinical case of PARP inhibitor resistance. Consequently, mutation of PARP1 DNA-binding zinc fingers has resulted in talazoparib resistance in ovarian cancer patients. Resistance to drugs is a major problem in the development of new classes of anticancer drugs; as new drugs are developed, cancer cells also develop resistance to them. Another mechanism of resistance that has been shown pre-clinically and during clinical studies is activation of the epithelial-mesenchymal transition (EMT). Ordonez et al. (2019) stated that EMT-like tumours are highly resistant to chemotherapies, and their findings indicated that EMT is linked to both intrinsic and acquired olaparib resistance. Additionally, resistance against olaparib is expected because olaparib was the first PARP1 inhibitor.Recently, studies have shown yet another mechanism of resistance to olaparib in aldehyde dehydrogenase-positive (ALDH+) cells that carry BRCA1 mutations. Gilabert et al. (2014) described findings of resistance to olaparib in ALDH+ cells from the BRCA1-mutated HCC1937 BCL, which was linked to a high level of PARP1 overexpression. One possible explanation for the finding is that PARP1 overexpression increases breast cancer stem cell (BCSC) DNA repair capacity, favouring resistance to DNA-damaging treatments like olaparib. Consequently, PARP1 inhibitors may not be effective, which means that damaged DNA would be repaired.The last mechanism of resistance of concern is the interaction of mesenchymal epithelial transition factor receptor (MET) and epidermal growth factor receptor (EGFR) with Y907 of PARP1. According to Dong et al. (2018), in the nucleus, the MET-EGFR heterodimer?interacts with and phosphorylates Y907 of PARP1, contributing to PARP inhibitor resistance in the context of hepatocellular carcinoma (HCC).?In summary, the aforementioned mechanisms of resistance have negative impacts on the treatment of cancer. Combining different classes of anticancer drugs or manipulating specific resistance-related genes may be possible solutions for countering drug resistance.The future of PARP1 inhibitorsMany studies have been conducted with the aim of devising solutions for the challenges of drug resistance that have been reported previously. First, Shen et al. (2019) recently discovered and evaluated a novel class of PARP1 inhibitors called naphthacemycins, as shown below in Figure 2. Figure 2. The general structure of naphtacemycinsThese were developed from a microbial metabolite library using enzyme-based high-throughput screening (HTS). The naphthacemycins inhibited PARP1 with IC50 values ranging from 0.95 ± 0.08 to 18.31 ± 0.34 ?M, as well as selective cytotoxicity towards BRCA1-deficient cells. Thus, the discovery of naphthacemycins as a new class of PARP1 inhibitors paves the way for the development of new anticancer drugs.Another novel approach that has been identified against PARP1 involves piperazine-based hits. According to Kader and Serdar (2021), Mol-1388 compounds are superior to currently approved PARP inhibitors in terms of selectivity against PARP1. Ten top-hit compounds have been identified with a docking score of 6.11 kcal/mol, compared to -14.92 kcal/mol for olaparib, indicating that they are more selective for PARP1 than olaparib. This new scaffold may open up new possibilities for developing small PARP1 inhibitors that can be used in cancer therapy by focusing on PARP1 selectivity.Another new class of PARP1 inhibitors, as reported by Makhov et al. (2020), involves the novel therapeutic modality of targeting the histone-dependent route of PARP1 enzyme activation, which could be a solution to overcome the limitation of NAD-like PARP inhibitors. NAD+ is used by many enzymes besides PARP1, and hence NAD+ competitors often have off-target effects. In vitro and in vivo studies have shown that modern histone-dependent PARP1 inhibitors are more effective against prostate and renal tumours than traditional NAD-like PARP1 inhibitors. Taken together, the results indicate that histone-dependent PARP1 inhibitors have significant therapeutic potential in the treatment of urological tumours when taken together.Discovering multiple ways for targeting the PARP1 enzyme allows for the development of new inhibitors that may be more effective or selective. Here, I discuss a new compound with higher PARP1 selectivity than the currently approved PARP1 inhibitors. Shuai et al. (2020) describes the design and synthesis of homoerathrina (Figure 3) as PARP1 inhibitors with promising results when compared with other drugs such as rucaparib. Figure 3. The general structure of the target compoundsThe results reveal that compound 10n, as shown below, inhibited the proliferation of A549 cells more potently (IC50 = 1.89 M) and with more selectivity when compared to rucaparib (IC50 = 4.91 ?M). In combination with the above findings, compound 10n inhibited PARP1 enzymatic activity more effectively, limiting PAR biosynthesis and resulting in improved efficacy in inhibiting A549 cell proliferation. This novel approach might result in the development of novel drugs that are better than those of the current compounds.The last strategy that can be used to overcome PARP1 resistance is combining PARP1 inhibitors with other agents. For instance, studies have shown that combinations of PARP1 inhibitors with HR agents may be useful in overcoming resistance to PARP1 inhibitors (Alan, 2018).In summary, the discovery of new classes of PARP1 inhibitors not only improves their activity or selectivity, but also prevents some mechanisms of resistance against the PARP1 inhibitor.ConclusionsIn conclusion, inhibiting the repair of DNA damage is one of the best ways to prevent cancer cells from replicating, which is exactly what PARP1 inhibitors do. Consequently, this essay has outlined the role of PARP1 inhibitors in anticancer therapy with regard to PARP1 targeting, the development of resistance to some PARP1 inhibitors and the future of PARP1 inhibitor development. Although only a few such drugs have been approved, several other compounds are undergoing preclinical studies and clinical trials. Additionally, these new compounds are not only a solution for overcoming drug resistance but can also signal the discovery of new targets for PARP1 that avoid the limitations of current NAD-like PARP inhibitors which compete with the NAD+ substrate for PARP binding.ReferencesAlan, D, D. 2018. Mechanisms of PARP inhibitor sensitivity and resistance. DNA Repair 71, pp. 172-176. https://doi.org/10.1016/j.dnarep.2018.08.021Christopher, L., J. and Alen, A. 2008. Targeted therapy for cancer using PARP inhibitors. 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