Targeted Therapies: PARPi

By Mark Comer

Traditional chemotherapy is often indiscriminate and common side effects of treatment arise from damage to non-malignant but rapidly proliferating cells, such as hair follicles and intestinal crypt cells. More profound is the resulting immunosuppressive effects of chemotherapy regimens which leaves patients at a higher risk of infection. These limitations of traditional chemotherapeutics represent a serious hurdle to cancer care. Recent advances in therapeutics focus on a more precise, targeted approach. Notably, the development and approval of the ‘miracle drug’ Imatinib (trade name: Gleevec) in 2001 which marked the beginning of so-called “targeted therapies” as a new therapeutic modality in the clinic. Several other drugs and treatments have been developed since then, including inhibitors of BRAF (a Raf kinase) and inhibitors of the enzyme poly ADP ribose polymerase (PARP). The increasing understanding of the genetics of cancer. alongside the application of CRISPR-Cas9 technologies is likely to continue to drive development of targeted therapies and usher in a new era of chemotherapies. However, targeted therapies may not provide a ‘silver bullet’ as they may fail to induce the long term responses necessary for a cure (Huang et al, 2019). Likewise, well characterised tumour suppressors and oncogenes are often considered “undruggable targets”, and resistance to targeted therapies is a subject of ongoing research (Huang et al, 2019). 

To distinguish targeted therapies more clearly from ‘classic’ chemotherapy drugs, an examination of Imatinib reveals the key differences. Imatinib is a tyrosine kinase inhibitor with high specificity for the constitutively active BCR-ABL fusion protein responsible for chronic myeloid and acute lymphoblastic leukaemia. Whilst it does have affinity for additional receptors, such as platelet-derived growth factor receptor and CD117, its mechanism is more precise than the prescribed non-targeted treatments such as Cytarabine, interferon infusions or alkylating agents. An examination of chronic myelogenous leukaemia outcomes following Imatinib’s approval reveals that 60 months following diagnosis, 89% of patients had survived (Druker et al, 2006). Prior to this, a mere 30% of patients survived 60 months following diagnosis and treatment with a non-targeted approach (Pray, 2008). Imatinib is an excellent example of the potential effectiveness of targeted therapies.

A comparatively significant development was the novel approach of utilising synthetic lethality in the clinic whereby simultaneous disruption of two genes would induce cell death. Applying the concept for anti-cancer purposes was first proposed by Hartwell et al in 1997. This was first demonstrated with PARP inhibitors targeting BRCA1 and BRCA2 mutations in breast cancers. BRCA1 and BRCA2 are tumour suppressor genes responsible for repair of double-stranded DNA breaks by complexing with Rad51 to perform homologous recombination (Scully, 2000)( Dziadkowiec et al,2017). Mutations in any component of DNA repair pathways can lead to genome instability and tumorigenesis, but cells possess redundancies in DNA repair pathways to prevent this. Women with a BRCA1 mutation have an approximately 50% chance of developing breast cancer by the age of 70 and those with a BRCA2 mutation have an approximately 40% chance (Chen et al, 2007); BRCA1/2 mutations also carry a risk of developing ovarian cancer (Chen et al, 2007).  Defective or unsuccessful base excision repair (BER) allows single strand breaks to develop into more damaging double strand breaks, and healthy cells will then employ homologous recombination (HR) or non-homologous end-joining (NHEJ) to correct the damage. BER is in turn mediated by PARP proteins. If repairs are unsuccessful, apoptosis induced by the p53 signalling pathway in response to DNA damage occurs. Subsequently, should PARP mediated repair be dysfunctional, a cell will ideally employ BRCA1 and BRCA2 to repair double strand breaks. Therefore, it follows that exploiting the relationship between defective BRCA1/BRCA2 in breast cancers and PARP proteins may serve as a means of developing a more specific therapy for patients.

PARP proteins are a family of 18 proteins that are key in BER (Amé, Spenlehauer, & de Murcia, 2004) and have an additional, but less important, function in repairing double strand breaks (Chen,2011). PARP1 binds single-stranded DNA breaks before generating poly-ADP ribose (PAR), transferring PAR to acceptor proteins such as histones and PARP1 itself. Transfer of PAR in turn aids in recruitment of additional repair proteins such as XRCC1 (Chen,2011). Hence, PARP inhibition in BRCA1/BRCA2 mutated breast cancers leads to selective cytotoxicity in cancer cells as these cells cannot accurately repair double strand breaks and employ more error-prone mechanisms. Accumulation of DNA damage then leads to apoptosis. Normal cells remain unaffected as they contain a functioning copy of BRCA1 or BRCA2 (Farmer et al, 2005). BRCA2-deficient cells in vitro were approximately 90 times more sensitive to PARP inhibition than wild-type cells, indicating an impressive level of specificity (Evers et al, 2008).  PARP inhibition was also approximately three times more potent than cisplatin in vitro (Farmer et al, 2005).

 PARP inhibitors have since been approved to treat a variety of BRCA1/BRCA2 mutated cancers including ovarian cancer. PARP inhibitors also boost the effects of DNA damaging agents such as cisplatin, cyclophosphamide and temozolomide (Donawho et al¸2007) in mice xenograft models. p53 deficient cancer cell lines which are traditionally resistant to DNA damaging drugs also became sensitised following treatment with PARP inhibitors – cytotoxicity was boosted as much as 2.3 times in vitro (Muñoz-Gámez et al, 2005).  

Identification of synthetically lethal “gene pairs” is likely to be key in future drug developments with application of CRISPR-Cas9 technology as the main approach for screening. The diverse genetic landscape found in tumours complicates the conceptually simple idea of synthetic lethality. Thus, success in the future requires careful study of the myriad of genetic interactions seen in cancer to identify gene interactions that can be disrupted for maximum therapeutic benefit (Huang et al, 2019). Undiscovered combination therapies may also provide a more desired long-term response in patients, whilst the definition of ‘undruggable targets’ shifts in response to development of new technologies, such as the use of proteolysis targeting chimeras (PROTACs) to degrade specific proteins. The landscape of cancer care has been fundamentally changed and targeted therapies herald in a new age of cancer care in the clinic.


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