Combining Neo-adjuvant ATR-inhibition With Nodal SBRT for Early-stage Resectable HPV+ OPSCCs

This research study is testing whether a new study drug (called ATRN-119) is safe and effective when combined with a single, highly targeted dose of radiation therapy (called stereotactic body radiation therapy or SBRT) to treat early-stage throat cancer that is caused by HPV. The goal of this study is to treat cancer effectively while reducing side effects and helping patients maintain a better quality of life over the long term. Researchers hope that this approach will be just as successful-or possibly more successful-than current treatments, which already have high cure rates. Participants will: * Take 800mg of ATRN-119 every day for 10 days * Receive 1 treatment (called a "fraction") of SBRT to the neck on day 3 of ATRN-119 dosing. * Keep a short diary to track ATRN-119 dosing. The diary will be provided by study team * Receive standard of care treatment after treatment with ATRN-119 + SBRT including TransOral Robotic Surgery (TORS) to remove the primary tumor with Neck Dissection and adjuvant therapy (if indicated) * Receive additional safety checkups and tests by researchers during routine visits with their cancer doctor for 2 years after study treatment

Inclusion Criteria: * Age ≥ 18 * Patients that have been diagnosed with HPV-positive throat cancer (OPSCC) at an early stage (stage I or II), confirmed by a biopsy. * Patients' blood calcium level is within a safe range * Patients' blood, kidney, and liver health are strong enough, shown through specific medical test results * If a participant is female, she must not be pregnant or breastfeeding. * If a participant is male, he must agree to use highly effective birth control from the start of the study until a short period after the last dose of the study drug. Exclusion Criteria: * Patients with late-stage tumors (Stage III or IV) * If the cancer has already spread to distant parts of the body (M1) when first diagnosed * Prior advanced head and neck cancer requiring radiation or major surgery * Patients whose original tumor site cannot be identified * Patients that have a known additional malignancy that is progressing or requires active treatment * Patients that had a surgical procedure performed within 7 days prior to first scheduled dose of ATRN-119. This does not include procedures that are used to diagnose or determine extent of study disease (such as biopsies). * Patients taking medications that strongly affect how the body processes certain drugs (CYP enzymes) at the same time as the study treatment. * Patients with active infections and/or receiving systemic antibiotics or anti-viral medications. * Patients with uncontrolled HIV or active hepatitis B or C are usually not eligible. However, those whose infections are well-controlled on treatment for at least a month and meet certain viral load limits can participate * Current or past diagnosis of leukemia within the past 5 years. * Patients with a history of non-malignant gastrointestinal (GI) bleeding, gastric stress ulcerations, or peptic ulcer disease within the past 3-months. * Patient has uncontrolled hypertension at time of enrollment. * Patients who recently had serious kidney problems or kidney disease. * Patients cannot have taken another experimental drug within 30 days-or within a period equal to five times that drug's half-life-before starting this study * Medical illness that, in the opinion of the Investigator, may impact the safety of the patient * Patients who use recreational drugs or have mental health conditions that might make it hard to follow study visits, based on the doctor's judgment. * Known hypersensitivity to ATRN-119 or its ingredients * Patients with serious liver disease or liver problems that could affect how the body processes the study drug * Patients who are fairly limited in daily activities (ECOG score of 2 or higher) * Patients with serious other health problems that the doctor thinks could prevent them from completing the study * Pregnancy or lactation * Inability to provide informed consent