Abstract: Radiotherapy is a cornerstone of cancer treatment, yet its efficacy is often limited by tumor radioresistance and the risk of toxicity to adjacent normal tissues. DNA polymerase theta (Polθ), an enzyme critical for microhomology-mediated end-joining (MMEJ) DNA repair, has emerged as a promising target for tumor-specific radiosensitization due to its overexpression in many cancers and minimal expression in normal tissues. ART899 is a novel, highly specific, small-molecule allosteric inhibitor of the Polθ DNA polymerase domain. Developed as a deuterated derivative with significantly improved metabolic stability over its predecessors, ART899 demonstrates potent radiosensitizing effects in preclinical models. It effectively impairs DNA double-strand break (DSB) repair, leading to increased genomic instability and cell death, particularly when combined with fractionated radiation. Importantly, ART899 exhibits excellent tumor specificity, showing no radiosensitizing effects on noncancerous cells. In vivo studies reveal that ART899 possesses a favorable pharmacokinetic profile, is well-tolerated upon oral administration, and significantly delays tumor growth when combined with fractionated radiotherapy. These findings highlight ART899 as a safe and effective radiosensitizer, paving the way for its clinical evaluation in solid tumors.
1. Introduction
Approximately 50% of cancer patients receive radiotherapy; however, survival rates following radical radiotherapy remain poor for many tumor types, and radiation delivered to adjacent normal tissues is frequently associated with significant side effects [1]. A major strategy to improve radiotherapy outcomes is to selectively increase the radiosensitivity of tumor cells without exacerbating the sensitivity of surrounding normal tissues. DNA polymerase theta (Polθ, encoded by the POLQ gene) represents an ideal target for this approach. Polθ is a DNA repair enzyme that plays a key role in microhomology-mediated end-joining (MMEJ), an error-prone DNA double-strand break (DSB) repair pathway [1]. Because Polθ has low or absent expression in most normal tissues but is frequently overexpressed in various cancer types, its inhibition offers a wide therapeutic window for tumor-specific radiosensitization [1]. ART899 is a novel, small-molecule allosteric inhibitor of the Polθ DNA polymerase domain designed to exploit this vulnerability and enhance the efficacy of radiotherapy [1].
2. Pharmacological Activity
ART899 demonstrates potent and specific pharmacological activity as a Polθ inhibitor. In cellular MMEJ assays, ART899 effectively inhibits MMEJ activity with an IC50 of approximately 180 nmol/L [1]. In vitro clonogenic survival assays show that ART899 effectively radiosensitizes human cancer cell lines, such as HCT116 (colorectal) and H460 (lung), particularly when combined with fractionated ionizing radiation (IR) (e.g., 5 × 2 Gy). At a concentration of 3 μmol/L, ART899 decreased survival in H460 cells by 5-fold and in HCT116 cells by 4-fold compared to radiation alone [1]. Crucially, ART899 exhibits high tumor specificity; it does not radiosensitize noncancerous human fibroblast lines (MRC5 and AG01552) or noncancerous epithelial cells (HIEC-6), nor does it affect Polθ knockout cells [1].
In vivo, ART899 displays a highly favorable pharmacokinetic profile, maintaining a plasma concentration of ≥1 μmol/L for several hours following oral administration in mice [1]. 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 mice displaying no significant weight loss, signs of distress, or treatment-related adversity [1].
3. Molecular Mechanism of Action
ART899 functions by specifically inhibiting the polymerase activity of Polθ, thereby blocking the MMEJ DNA repair pathway [1]. The radiosensitization caused by Polθ inhibition is driven by an impairment in DSB repair. Following irradiation, Polθ inhibition leads to a higher proportion of residual IR-induced DNA damage, evidenced by increased numbers of γH2AX and 53BP1 foci at 16 and 24 hours post-irradiation [1]. This failure to repair DNA breaks results in lethal chromosomal rearrangements and increased formation of IR-induced micronuclei, which are markers of genomic instability [1].
The mechanism is also highly cell-cycle dependent. MMEJ functions predominantly during the S and G2 phases of the cell cycle. Consequently, cells actively replicating (traversing the S phase) are more sensitive to Polθ inhibition than those in the G1 phase [1]. This cell-cycle selectivity explains why ART899 is particularly effective when combined with fractionated radiotherapy, as the time between radiation fractions allows cancer cells to progress into more radiosensitive phases of the cell cycle [1]. Additionally, Polθ inhibition successfully radiosensitizes tumor cells even under severe hypoxic conditions (<0.1% oxygen), a microenvironmental factor that typically confers significant radioresistance to solid tumors [1].
4. Structure-Activity Relationship (SAR)
ART899 was developed to overcome the pharmacokinetic limitations of earlier Polθ inhibitors. Its predecessor, ART558, demonstrated potent in vitro activity but was unsuitable for in vivo therapeutic use due to poor metabolic stability [1]. ART899 is an optimized derivative of ART558; specifically, it is a deuterated form of ART812 (an ART558 derivative) [1]. The strategic addition of deuterium during the synthesis of ART899 resulted in greatly improved metabolic stability. Clearance values obtained using mouse and rat liver microsome stability assays confirmed that ART899 has significantly lower intrinsic clearance compared to ART558, enabling its successful oral administration and sustained plasma concentrations in vivo [1].
5. Current Limitations
While ART899 shows significant promise, a current limitation in its preclinical evaluation relates to the physiological complexities of solid tumors. Although ART899 effectively radiosensitizes cells under hypoxic conditions in vitro, tumor hypoxic regions in vivo are typically characterized by limited blood perfusion. Therefore, further investigation is required to establish whether this family of Polθ inhibitors can achieve and maintain sufficiently high drug concentrations within these poorly perfused hypoxic tumor microenvironments to induce effective radiosensitization in patients [1].
6. Future Perspectives
The preclinical success of ART899 broadens the potential therapeutic application of Polθ inhibitors beyond their initial use as a standalone synthetic lethal therapy for tumors with homologous recombination (HR) defects (such as BRCA mutations) [1]. By demonstrating safe and effective tumor-specific radiosensitization in HR-proficient models, ART899 paves the way for combining Polθ inhibitors with standard fractionated radiotherapy across a wider range of solid tumors. These promising results support ongoing clinical translation; a first-in-human clinical trial (NCT04991480) is currently evaluating the safety and activity of Polθ inhibition in patients with solid tumors, which may open new clinical avenues for overcoming radioresistance and improving patient survival [1].
