DNA damage repairing, a cellular guardianship of genomic stability, involves complex molecular machinery, including mismatch repair (MMR), nucleotide excision repair (NER), base excision repair (BER), and double-strand break repair (DSB repair). Exhaustive explorations have been conducted in the last two decades to glean mechanistic insights into DNA damage response pathways. The disclosures from such explorations have provided utmost clarity that these pathways repair the single/double-strand DNA break and regulate the cell-cycle progression and cellular functioning [1,2]. Any deception in DNA repair or impairment in the repairing orchestras diminishes the genomic stability and transforms the cells into cancer cells [[1], [2]]. Although the DNA repair deficiencies contribute to tumorigenesis, such defects are also conceived as potential therapeutic opportunities for cancer treatment as tumour cells are more susceptible to DNA damage than normal cells and are dependent on specific functional repair pathways for survival [3,4]. In light of deficient DNA response being more deleterious in tumour cells than normal cells, adding tractable entries to the compendium of DNA damage repair pathway inhibitors is presently considered to be a sagacious approach to expand the therapeutic landscape in oncology.

Poly (ADP-ribose) polymerases (PARPs) are a family of proteins that promote the repair of DNA single-strand breaks on activation by DNA damage via the addition of branched Poly(ADP-ribose) (PAR) chains. As such, there are 17 PARP family members expressed in humans, each with a unique structural domain, function, and subcellular location [5]. Among all members, PARP1 and PARP2 are major enzymes that regulate the DNA damage response in cancer [6]. Notably, overexpression of PARP1 and 2 is observed in several cancer types, including breast, lung, ovarian, uterine cancer, and acute myeloid leukemia [[7], [8], [9], [10]]. Ascertainment of the multifaceted involvement of PARPs in cancer has spurred intensive research efforts, culminating in the development of effective PARP-based therapeutics to inhibit DNA repair pathways and cause apoptosis of cancer cells. Resultantly, several PARP inhibitors have been approved by the FDA and a plethora of small molecule structural templates are undergoing clinical stage evaluations. Olaparib, rucaparib, niraparib, talazoparib and fuzuloparib (CFDA approved) represents the FDA/CFDA approved PARP inhibitors while veliparib, stenoparib, mefuparib, atamparib, saruparib, and pamiparib (Fig. 1) have strengthened the armory of investigational PARP inhibitors.

The progress chart of PARP inhibitors perspicuously underscores their potential in blocking the DNA repairing mechanism; however, a therapeutic profile plagued with limitations has restricted their clinical utility. Despite an effective blockade of the DNA repair mechanism by PARP inhibitors, the second line of defence (homologous recombination – a well-known mechanism for fixing double-strand breaks) can save the cells. Owing to this, PARP inhibitors are labelled as “therapeutics with narrow activity spectrum” as they exploit the concept of synthetic lethality that selectively relies on the vulnerability of tumour cells with defects in DNA damage repair (DDR) pathways and occurs when the perturbation of two genes occurs simultaneously [11]. Thus, PARP inhibitors manifest efficacy only in tumours harbouring the BRCA mutations (homologous recombination deficiency). The aforementioned has prompted the push to outwit the limitations of PARP inhibitors in the quest to extract the antitumor potential of this exciting class of targeted anticancer drugs. Literature precedents indicate that BRCAness induction via targeting additional enzymes/receptors/proteins to directly or indirectly disturb the homologous recombination (HR) repair pathway can lead to the expansion of the application horizons of PARP inhibitors [12]. Thus, the assemblage of dual modulatory chemical architectures is presently being conceived as a logical strategy to overcome the deficiencies of PARP inhibitors as cancer therapeutics.

Histone Deacetylases (HDACs) are the key epigenetic regulators that catalyze the removal of an acetyl group from histone and non-histone proteins and regulate gene expression by maintaining the transcriptional equilibrium with histone acetyltransferases (HATs). The nuanced balance, governed by HDACs and HATs, allows post-translational modification by modulating the chromatin structure for precise transcriptional activation or repression and shapes the epigenetic landscape for cellular functioning. It has been evidenced that dysregulation in HDAC-mediated post-translational modification or aberrance in the expression of HDAC directly contributes to cancer development. In addition, HDACs are also directly involved in cancer cell proliferation, metastasis, angiogenesis, apoptosis, cell cycle, and inflammatory processes [13]. Given the influence of HDACs on the critical pathways of cancer cell development, HDAC inhibition was leveraged as an imperative strategy for the design of small molecule antitumor scaffolds in the last two decades. Resultantly, several HDAC inhibitors were approved by FDA/CFDA as anticancer agents viz. vorinostat, belinostat, romidepsin, panobinostat and chidamide. Also, a investigation HDAC inhibitor, MS-275, has demonsteted promising results in diverse malignancies (Fig. 1) [14]. Lately, the endeavors on HDAC inhibitors have been directed towards the design of selective inhibitors of HDAC isoforms to attain toxicity free HDAC inhibitors and HDAC targeting chimeric adducts (dual inhibitors). Delightfully, bifunctional HDAC therapeutics have manifested magnificent outcomes in diverse malignancies [15], and this has led to the transposition in the inclination of researchers from “single target HDAC inhibitors” to “multi-target agents”.

Histone deacetylase (HDAC) inhibition, in addition to eliciting antitumor effects, is a known inducer of pharmacologic BRCAness in cancer cells with proficient DNA repair activity. In light of the aforestated, numerous combinations of PARP and HDAC inhibitors have been screened to tease out synergistic efficacy and broaden the activity spectrum of PARP inhibitors [[16], [17], [18], [19], [20], [21]]. Of note, the favourable outcomes attained with combination therapies are often utilized as inception points for the design of dual inhibitors strategically engineered to target two different biological pathways and extract enhanced therapeutic effects with a single chemical entity. Despite both approaches, combination therapy and dual inhibition, aim to attain synergistic antitumor efficacy and alleviate the deficiency of single drugs, the latter is presently considered to be a superior stratagem that scores over the former in terms of simpler pharmacokinetics. The above-mentioned extensive insights propelled us to embark on a medicinal chemistry endeavour in pursuit of leveraging PARP-HDAC as a compelling target for the construction of anti-tumour adducts. Accordingly, dual PARP-HDAC inhibitors were designed, synthesized and profiled as antitumor agents in this study.

In addition to meticulous crafting and unearthing the underpinnings of dual PARP-HDAC inhibitors as antitumor agents, attempts in this study were also directed towards the design of a pH-responsive nanoformulation. Noteworthy to mention that our research group has recently initiated a campaign where the outcome of a medicinal chemistry program, a promising anticancer chemical architecture, serves as a starting material for the construction of pH-responsive nanoparticles [22]. The aforementioned embarkment stemmed from the intrigues aroused by the poor translation rate of HDAC inhibitors (monofunctional/bifunctional) from preclinical to clinical studies owing to off-target effects and systemic toxicity. The aforestated limitation of HDAC inhibitors can be outwitted by the development of nanocarriers (stimuli responsive) that can ensure controlled drug release triggered by the acidic pH of the tumour microenvironment. In particular, polymer nanoformulations have demonstrated significant promise in delivering small molecule inhibitors at the cancer site in a targeted manner leading to their intra-tumoral accumulation [[23], [24], [25]]. Spurred by the aforementioned, this pursuit is also aimed at improving the tumour-specific accumulation of the most efficacious scaffold pinpointed through a series of in-vitro experiments that showcased its prowess as an antitumor assemblage in this study.