Inclusion Criteria:

1. Provide written informed consent.
2. 18 years old or older.
3. Advanced, metastatic, and/or progressive solid tumors for whom there is no authorized or effective therapy available, or for whom such therapies are considered inappropriate by the Investigator.
4. Measurable disease according to Response Evaluation Criteria in Solid Tumors (RECIST) Version 1.1.
5. Eastern Cooperative Oncology Group (ECOG) performance status of 0-1.

Exclusion Criteria:

1. Subjects with known human epidermal growth factor receptor 2 (HER2+) overexpressing malignancies.
2. Radiotherapy (except for palliative reasons), endocrine therapy, chemotherapy, or investigational agent within 28 days, (42 days for nitrosoureas, mitomycin-C) of first dose of NXP900. Subjects can continue to receive bisphosphonates due to metastatic bone disease or GnRH agonists if they have prostate cancer.
3. Ongoing toxic manifestations of previous treatments > Grade 2 with the exception of alopecia and neuropathy.
4. Subjects with treated brain metastases with evidence of progression within 28 days after central nervous system (CNS)-directed treatment, as ascertained by clinical examination and brain imaging (magnetic resonance imaging [MRI] or computed tomography [CT] scan) during the Screening period.
5. Female subjects who can become pregnant (or are already pregnant or lactating), unless they have a negative serum pregnancy test before enrollment and agree to use at least one highly effective form of contraception .
6. Male subjects with partners of childbearing potential, unless they agree to take measures not to father children by using a barrier method of contraception (condom plus spermicide).
7. Major surgery from which the subject has not yet recovered.

Note: Other protocol defined Inclusion/Exclusion Criteria may apply.

Eligibility last updated 7/1/23. Questions regarding updates should be directed to the study team contact.