A Study of Mirdametinib on Its Own or in Combination With Fulvestrant in People With Solid Tumor Cancer

Inclusion Criteria: Subjects are eligible to start the treatment in the study only if all of the following criteria apply: Arm 1 (metastatic breast cancer): * Female adults (≥18 years of age on the date of informed consent) * Postmenopausal or receiving ovarian suppression (including GnRH agonists such as goserelin) * Histologically confirmed hormone receptor-positive metastatic breast cancer with evidence of progression on at least 1 prior line of therapy for metastatic disease which should have included a CDK4/6 inhibitor in combination with endocrine therapy. Prior chemotherapy is permitted. * ER+ as defined by immunohistochemistry (IHC) ≥1% by local laboratory testing (as per the ASCO-CAP guidelines) * HER2-negative, as defined by the negative in situ hybridization test (FISH, CISH, or SISH) or an IHC status of 0, 1+ or 2+ by local laboratory testing. If IHC is 2+ (i.e. indeterminate), then a negative in situ hybridization test (FISH, CISH, or SISH) is required (as per the ASCO-CAP guidelines). * NF1 loss of function or another MAPK-activating genomic alteration documented by a CLIA-certified NGS assay at any time before the start of treatment. Note: archival tissue, if available and collected within 6 months of enrollment, may be used for this testing in lieu of fresh tissue. Arm 2 (advanced solid cancers): * Male or female adults (≥18 years of age on the date of informed consent) * Histologically confirmed advanced, metastatic solid tumor cancer for which there i…