The goal of this clinical trial is to learn about what doses of BTM-356 are safe to use in adults with advanced cancers in solid tumors. It will also learn about how effective different doses of BTM-3566 are in treating cancer. The main questions it aims to answer are:

What adverse events and toxicities (harmful side effects) are associated with different doses of BTM-3566? What are the blood levels of BTM-3566 in your body at different timepoints? What effect does BTM-3566 have on reducing tumor size and/or preventing the worsening of cancer? All participants will receive BTM-3566 and none will receive placebo (a look-alike substance that contains no drug).

Participants will:

• Take BTM-3566 for 7 days, then take a 7 day break. This 2 week dosing schedule of 7 days on/7 days off will continue until your disease worsens or you cannot tolerate treatment.
• Visit the clinic approximately 7 times during screening and the first month of treatment. Visits will be reduced to approximately 2 visits per month after.
• Keep a diary of when (and how much) BTM-3566 you take, along with how much you weigh on each dosing day.

This study is a phase 1, open label, dose escalation study using single participant cohorts followed by 3+3 design to evaluate multiple ascending doses of BTM-3566.

Key Inclusion Criteria:

• Any advanced, unresectable and/or metastatic solid tumors except for primary brain cancer, breast cancer, prostate cancer, pancreatic cancer, pheochromocytoma, fibrolamellar carcinoma, adrenocortical carcinoma, or ocular or cutaneous melanoma.
• Must be refractory/relapsed after all standard therapies known to provide proven clinical benefit
• ECOG Performance Status 0 to 2
• Adequate organ function as defined by pre-specified laboratory values
• Life expectancy > 3 months
• Women of child-bearing potential (or males with female partners of child-bearing potential) must agree to use adequate contraceptive measures throughout the study and for 90 days following last dose

Key Exclusion Criteria:

• Has not completed appropriate wash-out timeframes of prior anti-cancer treatments
• Has ongoing toxicities from prior anti-cancer treatments
• Has symptomatic or uncontrolled neurologic disease
• Has active and clinically significant bacterial, fungal or viral infection
• Cannot avoid use of moderate or strong CYP3A4 inhibitors or inducers; CYP2C19 substrates or OAT2 substrates with narrow therapeutic index; or drugs that prolong the QT interval throughout the study
• Has previously received a total anthracycline dose ≥ 360mg/m2 doxorubicin or equivalent
• Has a history of serious cardiac conditions or pulmonary or cerebrovascular events within 6 months of first dose
• Is pregnant or breastfeeding

This is an exploratory proof of concept study with no formal hypothesis testing and may not be suitable for publication in a peer-reviewed journal.