Abstract: Radiotherapy is a cornerstone of cancer treatment, yet its efficacy is frequently limited by tumor radioresistance, particularly induced by hypoxia, and the risk of toxicity to adjacent normal tissues. DNA polymerase theta (Polθ), a key enzyme in the microhomology-mediated end-joining (MMEJ) DNA repair pathway, has emerged as a promising tumor-specific target due to its overexpression in cancer cells and minimal presence in normal tissues. This review explores the pharmacological profile of ART899, a novel, highly specific, and metabolically stable small-molecule inhibitor of Polθ. ART899 demonstrates potent radiosensitizing effects in both normoxic and severely hypoxic conditions, effectively overcoming hypoxia-induced radioresistance. By impairing double-strand break (DSB) repair and increasing genomic instability without affecting noncancerous cells, ART899 combined with fractionated radiotherapy offers a safe and highly effective strategy for tumor growth delay, paving the way for future clinical applications.
1. Introduction
Approximately half of all cancer patients receive radiotherapy; however, survival rates following radical radiotherapy remain suboptimal for many solid tumor types due to intrinsic and microenvironmental resistance factors [1]. A major microenvironmental factor contributing to treatment failure is tumor hypoxia, as hypoxic cells can be over three times more radioresistant than normoxic cells [1]. Furthermore, the maximum radiation dose that can be delivered is often limited by the potential for significant side effects in surrounding healthy tissues [1]. To improve radiotherapy outcomes, strategies are needed to selectively increase the radiosensitivity of tumor cells without exacerbating normal tissue toxicity.
DNA polymerase theta (Polθ, encoded by the POLQ gene) represents an ideal tumor-specific radiosensitization target. It is a DNA repair enzyme that is frequently overexpressed in various cancer types but exhibits low or absent expression in most normal tissues [1]. While Polθ deficiency is known to be synthetically lethal with homologous recombination (HR) defects (such as BRCA mutations), recent evidence suggests that pharmacological inhibition of Polθ can radiosensitize a wide range of tumors irrespective of their HR status [1]. This has led to the development of novel small-molecule Polθ inhibitors, such as ART899, designed to enhance the efficacy of radiotherapy and reverse hypoxia-induced radioresistance [1].
2. Pharmacological Activity
ART899 is a potent and specific allosteric inhibitor of the Polθ DNA polymerase domain [1]. In vitro, ART899 effectively radiosensitizes HR-proficient cancer cell lines, such as HCT116 (colorectal) and H460 (lung), particularly when combined with fractionated ionizing radiation (IR) schedules (e.g., 5 × 2 Gy) [1]. At a concentration of 3 μmol/L, ART899 decreased the survival of H460 and HCT116 cells by 5-fold and 4-fold, respectively, compared to radiation alone [1]. Importantly, ART899 exhibits excellent tumor specificity; it does not radiosensitize noncancerous human fibroblast lines (MRC5 and AG01552) or the noncancerous epithelial cell line HIEC-6 [1].
In the context of hypoxia-induced radioresistance, Polθ inhibition has proven highly effective. Experiments conducted under severe hypoxia (<0.1% oxygen) demonstrated that Polθ inhibitors confer significant radiosensitization, yielding Sensitization Enhancement Ratios (SER10) only slightly lower than those observed under normoxic conditions [1]. This positions ART899 as a promising agent to overcome the radioresistance typically conferred by the hypoxic tumor microenvironment.
In vivo, ART899 demonstrates a favorable pharmacokinetic profile, maintaining plasma concentrations ≥1 μmol/L for several hours following oral administration in mice [1]. When tested in HCT116 subcutaneous xenograft models, the combination of oral ART899 (150 mg/kg twice daily) with fractionated irradiation (10 × 2 Gy) significantly improved tumor growth delay compared to radiation alone [1]. Furthermore, the combination treatment was well-tolerated, with no significant weight loss or treatment-related adverse effects observed, confirming its safety and efficacy in preclinical models [1].
3. Molecular Mechanism of Action
Polθ plays a critical role in microhomology-mediated end-joining (MMEJ), an error-prone DNA double-strand break (DSB) repair pathway that relies on short homologous sequences (2–4 bp) across the break site [1]. ART899 specifically inhibits cellular MMEJ activity with an IC50 of approximately 180 nmol/L [1]. The radiosensitizing effect of ART899 is strictly Polθ-dependent, as evidenced by the lack of radiosensitization in Polθ knockout (KO) cells and cells subjected to siRNA-mediated Polθ depletion [1].
Mechanistically, Polθ inhibition does not increase the initial induction of DSBs at the time of radiation but rather impairs subsequent DNA damage repair. This impairment leads to a significantly higher number of residual IR-induced γH2AX and 53BP1 foci at 16 and 24 hours post-irradiation [1]. Consequently, the failure to repair DSBs results in lethal chromosomal rearrangements, visually confirmed by an increased formation of IR-induced micronuclei in treated cells [1].
Additionally, the radiosensitization induced by Polθ inhibition is cell-cycle dependent. MMEJ functions predominantly during the S and G2 phases of the cell cycle. Cells traversing the actively replicating S phase are inherently more radioresistant but are significantly more susceptible to Polθ inhibition [1]. Because fractionated radiotherapy allows cancer cells to progress through the cell cycle between radiation doses, it increases the likelihood of cells being irradiated during these sensitive phases, thereby maximizing the therapeutic synergy between ART899 and fractionated IR [1].
4. Structure-Activity Relationship (SAR)
ART899 was developed to overcome the pharmacokinetic limitations of its predecessor, ART558. While ART558 demonstrated potent in vitro activity, it was unsuitable for in vivo therapeutic use due to poor metabolic stability [1]. ART899 is an optimized, deuterated derivative of ART812 (a structural analog of ART558) [1]. The strategic addition of deuterium during the synthesis of ART899 resulted in a compound with greatly improved metabolic stability. Microsome stability assays revealed that ART899 possesses significantly lower intrinsic clearance values in both mouse and rat liver microsomes compared to ART558, while retaining comparable on-target potency (IC50 ~180 nmol/L) and specificity for the Polθ polymerase domain [1].
5. Current Limitations
While ART899 successfully addresses the metabolic instability of earlier Polθ inhibitors like ART558, challenges remain regarding its application in the tumor microenvironment. Hypoxic regions within solid tumors are typically characterized by limited vascular perfusion [1]. Although ART899 is highly effective at radiosensitizing hypoxic cells in vitro, further in vivo and clinical investigations are required to establish whether this class of small-molecule inhibitors can reliably penetrate and achieve therapeutically effective concentrations within these poorly perfused, severe hypoxic zones in human tumors [1].
6. Future Perspectives
The preclinical success of ART899 highlights the broad potential of Polθ inhibitors as universal radiosensitizers, extending their utility beyond the niche of HR-deficient (e.g., BRCA-mutated) tumors to HR-proficient cancers [1]. The ability of ART899 to safely enhance the effects of fractionated radiotherapy and reverse hypoxia-induced radioresistance provides a strong rationale for clinical translation. Currently, the therapeutic modality of Polθ inhibition is being evaluated in a first-in-human clinical trial (NCT04991480) assessing safety and activity in patients with solid tumors [1]. Future clinical studies will likely explore the integration of stable Polθ inhibitors like ART899 with standard-of-care radiotherapy regimens, potentially offering a breakthrough in overcoming radioresistance and improving survival outcomes for patients with hard-to-treat solid tumors [1].
