This first-in-human (FIH) study will test the safety and side effects of a drug called XMT-1660. A side effect is anything a drug does to the body besides treating the disease.
Participants in the study will have cancer that has come back after a period of time during which the cancer could not be detected (recurrent), spread in the body near where it started (advanced) or spread through the body (metastatic).
The study will have two parts. The first part called Dose Escalation will find out how much XMT-1660 should be given to participants. The second part called Dose Expansion will use the dose found in the first part to find out how safe XMT-1660 is and if it works to treat solid tumor cancers.
Inclusion Criteria:
* Recurrent or advanced solid tumor and has disease
* Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1
* Participants in DES must have at least one non-target lesion as defined by RECIST version 1.1. Participants in Backfill Cohorts and EXP must have at least one measurable disease (target) lesion as defined by RECIST version 1.1.
* Tumor tissue, either archival or from a fresh tumor biopsy, available for testing or be willing to undergo a minimally invasive tumor biopsy to obtain tumor tissue for local testing, if not medically contraindicated, prior to Cycle 1 Day 1
* Brain magnetic resonance imaging (MRI) during the Screening period unless obtained within 30 days prior to Screening (based on standard clinical care), if they meet either of the following criteria:
1. All participants with TNBC
2. Participants with a history of brain metastases or with neurologic symptoms or signs suspicious for brain metastases.
Exclusion Criteria:
* Prior treatment with an Antibody Drug Conjugate (ADC) containing an auristatin payload. Prior treatment with another ADC containing other payloads is allowed.
* Major surgery within 28 days of starting study treatment, systemic anticancer therapy within the time period of 28 days or 5 half-lives of the prior therapy before starting study treatment (14 days or 5 half-lives for small molecule targeted therapy), whichever is less, or palliative radiation therapy to the chest within 3 months of starting study treatment or to other anatomic sites within 14 days of starting study treatment.
* Diagnosis of additional malignancy that required active treatment (including surgery, systemic therapy, and radiation) within 2 years prior to screening, except for adequately treated basal cell or squamous cell skin cancer, or carcinoma in situ of the breast or of the cervix.
* Untreated CNS metastases (including new and progressive brain metastases), history of leptomeningeal metastasis or carcinomatous meningitis.
* Prior B7-H4 targeted treatment.
* History of cirrhosis, hepatic fibrosis, esophageal or gastric varices, or other clinically significant liver diseases.
* Current severe, uncontrolled systemic disease (e.g. clinically significant cardiovascular, pulmonary, or metabolic disease) or intercurrent illness that could increase the risk of serious adverse events (SAEs) or interfere with per-protocol evaluations, in the judgment of either the Sponsor or the Investigator.
* Clinically significant cardiovascular disease
