Berzosertib and Irinotecan in Treating Patients With Progressive, Metastatic, or Unresectable TP53 Mutant Gastric or Gastroesophageal Junction Cancer

Patients must have histologically or cytologically confirmed progressive metastatic or unresectable gastric or GEJ adenocarcinoma.

Patients must have measurable disease, defined as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded for non-nodal lesions and short axis for nodal lesions) as \>= 20 mm (\>= 2 cm) by chest x-ray or as \>= 10 mm (\>= 1 cm) with computed tomography (CT) scan, magnetic resonance imaging (MRI), or calipers by clinical exam.

Patients must have progressed after or been intolerant of at least two lines of systemic therapy. Patients with HER2 positive gastric and GEJ adenocarcinoma must have progressed on trastuzumab plus chemotherapy in the first line setting. Patients with microsatellite unstable (MSI-H) tumors must have received prior immunotherapy with pembrolizumab.

Age \>= 18 years. Because no dosing or adverse event data are currently available on the use of M6620 in combination with irinotecan in patients \< 18 years of age, children are excluded from this study, but will be eligible for future pediatric trials.

Both men and women of all races and ethnic groups are eligible for this trial.

Eastern Cooperative Oncology Group (ECOG) performance status =\< 1 (Karnofsky \>= 60%).

Leukocytes \>= 3,000/mcL.

Absolute neutrophil count \>= 1,500/mcL.

Platelets \>= 100,000/mcL.

Hemoglobin \>= 9 g/dL.

Total bilirubin within normal institutional limits.

Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase \[SGOT\])/alanine aminotransferase (serum glutamic pyruvic transaminase \[SGPT\]) =\< 2.5 x institutional upper limit of normal (ULN); if liver involvement =\< 5 x ULN.

Creatinine clearance \>= 45 mL/min/1.73 m\^2 for patients with creatinine levels above institutional normal.

Patients must have a TP53 mutation (only those known hot-spot mutations that fall within exon 2 or exons 4-11 will be accepted) determined from available archived tumor tissue that has been subjected to next generation sequencing (NGS) through FoundationOne/FoundationOneCDx or a similar assay performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified laboratory. Investigators from other sites, who have potential patients who meet study eligibility, will send copies of NGS reports from these patients via Medidata Rave case reports to the responsible study coordinator. Our research team will review each report to ensure each patient possesses the mutations of interest. Similar review will happen for each patient we enroll on the study at our institution. Case reports from all screened patients will be centrally available on the Rave study database.

Nine patients must be willing to undergo endoscopic or CT guided tumor biopsies for mandatory correlative studies. If the biopsy is deemed not safe by the treating physician, the patient may still enroll given that the other eligibility criteria are met.

The effects of M6620 on the developing human fetus are unknown. For this reason and because deoxyribonucleic acid (DNA)-damage response (DDR) inhibitors, as well as irinotecan, are known to be teratogenic, women of child-bearing potential and men able to father children who have female partners of child-bearing potential must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 6 months after trial participant's final dose of M6620 or irinotecan (whichever agent is completed last). Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately.

Ability to understand and the willingness to sign a written informed consent document.

What this study is about