Inclusion Criteria:

          -  Patients must have histologically confirmed metastatic or unresectable malignancy that
             is refractory to standard therapy or for which no standard therapy exists and where
             irinotecan is deemed a reasonable treatment option

          -  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) with
             conventional techniques or as >= 10 mm (>= 1 cm) with spiral computed tomography (CT)
             scan, magnetic resonance imaging (MRI), or calipers by clinical exam

          -  No limit on prior lines of therapy for metastatic disease; prior adjuvant or
             neoadjuvant chemotherapy does not count as a prior line of therapy as long as
             completion of the adjuvant or neoadjuvant therapy was more than 1 year prior to
             patient enrollment

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 (Karnofsky >= 70%)

          -  Life expectancy of greater than 12 weeks

          -  Leukocytes >= 3,000/mcL

          -  Absolute neutrophil count >= 1,500/mcL

          -  Platelets >= 100,000/mcL

          -  Total bilirubin within normal institutional limits

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT])
             =< 2.5 x institutional upper limit of normal (ULN); if liver involvement, =< 5 x ULN

          -  Creatinine clearance >= 60 mL/min/1.73 m^2

          -  Patients must have archived tumor tissue from prior tumor biopsy or surgical
             resections available for submission that is sufficient to complete molecular profiling

          -  FOR PATIENTS ENROLLED IN THE EXPANSION COHORT: willingness to undergo mandatory
             biopsies (day -14, approximately 4 hours post end of irinotecan infusion and day 1,
             approximately 4 hours post end of irinotecan infusion [= 3 hours post end of M6620]);
             patients enrolled to this cohort should have tumors deemed easily accessible for
             biopsies with low likelihood of complication

          -  The effects of M6620 on the developing human fetus are unknown; for this reason and
             because deoxyribonucleic acid (DNA)-damage response (DDR) inhibitors may have
             teratogenic potential, women of child-bearing potential and men must agree to use
             adequate contraception (hormonal or barrier method of birth control; abstinence) prior
             to study entry and for the duration of study participation; 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; men treated or enrolled
             on this protocol must also agree to use adequate contraception prior to the study, for
             the duration of study participation, and 6 months after completion of M6620
             administration

               -  For this reason and because DNA-damage response (DDR) inhibitors may have
                  teratogenic potential, women of child-bearing potential and men 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 study completion

          -  Ability to understand and willingness to sign a written informed consent document

        Exclusion Criteria:

          -  Patients who have had chemotherapy or other systemic therapy or radiotherapy or
             patients who have not recovered from adverse events due to prior administered agents
             as follows:

               -  Chemotherapy < 4 weeks prior to entering the study

               -  Radiotherapy < 4 weeks prior to entering the study

               -  Nitrosoureas/mitomycin C < 6 weeks prior to entering the study

               -  Targeted therapy < 2 weeks (or 5 half-lives, whichever is longer) prior to
                  entering the study

               -  Those who have not recovered from clinically significant adverse events due to
                  prior agents administered to grade =< 1 or baseline, with exception of alopecia
                  and peripheral neuropathy, unless approved by the protocol chair

               -  Immunotherapy < 4 weeks prior to entering the study

          -  Patients who are receiving any other investigational agents

          -  Patients with unstable brain metastases should be excluded; however, patients with
             known brain metastases may participate in this clinical trial if they are clinically
             stable (without evidence of progression by imaging for at least four weeks prior to
             the first dose of trial treatment and any neurologic symptoms have returned to
             baseline), have no evidence of new or enlarging brain metastases, and are on a stable
             or decreasing dose of steroids for at least 14 days prior to trial treatment

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to M6620 or irinotecan

          -  M6620 is primarily metabolized by CYP3A4; irinotecan and its active metabolite, SN-38,
             are metabolized by CYP3A4 and UGT1A1, respectively; therefore, concomitant
             administration with strong inhibitors or inducers of CYP3A4 should be avoided;
             valproic acid is known to inhibit the process of glucuronidation and may potentially
             enhance the toxicity of irinotecan; medications that enhance glucuronidation (i.e.
             phenytoin, phenobarbital, carbamazepine, rifampin, etc.) may also enhance clearance of
             SN-38, which may possibly decrease efficacy; therefore, concomitant administration of
             these drugs should be avoided; because the lists of these agents are constantly
             changing, it is important to regularly consult a frequently-updated medical reference
             for a list of drugs to avoid or minimize use of; as part of the enrollment/informed
             consent procedures, the patient will be counseled on the risk of interactions with
             other agents, and what to do if new medications need to be prescribed or if the
             patient is considering a new over-the-counter medicine or herbal product

          -  Uncontrolled intercurrent illness including, but not limited to, severe active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, or psychiatric illness/social situations that would limit compliance with
             study requirements; patients with chronic viral hepatitis may participate in this
             clinical trial if they are clinically stable with acceptable liver function

          -  Pregnant women are excluded from this study because M6620 as a DNA-damage response
             (DDR) inhibitor may have the potential for teratogenic or abortifacient effects;
             because there is an unknown but potential risk for adverse events in nursing infants
             secondary to treatment of the mother with M6620, breastfeeding should be discontinued
             if the mother is treated with M6620; these potential risks may also apply to other
             agents used in this study

          -  Human immunodeficiency virus (HIV)-positive patients with well-controlled disease, as
             determined by CD4 count and viral load, who are on antiretroviral therapy that does
             not contain a strong inducer or inhibitor of CYP3A4 (e.g. regimens containing
             ritonavir, cobicistat, efavirenz or etravirine) are allowed on trial; HIV-positive
             patients on combination antiretroviral therapy with strong inducers or inhibitors of
             CYP3A4 are ineligible because of the potential for pharmacokinetic interactions with
             M6620; patients with poorly controlled HIV are not eligible due to the increased risk
             of lethal infections when treated with marrow-suppressive therapy