Mirdametinib Monotherapy in Adults With Neurofibromatosis 1 (NF1) and Cutaneous Neurofibromas (cNF).

Inclusion Criteria: 1. Meet the diagnostic criteria for NF12 2. ≥ 18 years of age 3. Have a minimum of 24 measurable cNF (2 target areas of ≥6 measurable cNF) a. Measurable is defined as: i. non-pedunculated (no stalk) ii. surrounded by uninvolved skin and not adjacent to another cNF lesion iii. measuring ≥ 0.5 cm in the longest diameter and ≥ 0.5 cm in height iv. the 24 cNF must be located in two Target Areas. One target area must be located on the back and must have at least 6 measurable cNF. The second target area can be in any of the following body regions with at least 6 cNF: head and neck; upper extremities; anterior chest wall, anterior abdominal wall; pelvic region/gluteal region; lower extremities. 4. Participants must have cNF that meet eligibility criteria located within the two study designated target areas or outside of the target areas amenable to biopsy. If biopsy is taken within the target area there must be a minimum of 6 cNF remaining for long term surveillance after biopsy. Participants must be willing to undergo pre-, and on-treatment tumor biopsies providing fresh tumor tissue; there should be no contraindication for serial biopsy 5. Karnofsky performance level of ≥ 80%. 6. Adequate organ and bone marrow function as defined by the following Screening laboratory values: 1. Absolute neutrophil count ≥ 1500 cells/µL; 2. Platelets ≥ 100 x 103/µL; 3. Hemoglobin ≥ 9.5 g/dL; 4. Serum albumin ≥ 2.8 g/dL; 5. Calculated creatinine clearance at Screening ≥ 60 mL/min (by Cockcroft-Gault formula) OR a normal serum creatinine. 7. Participant is willing and able to comply with all aspects of the protocol 8. Ability to understand and the willingness to sign written informed consent document(s). 9. Women of childbearing potential (WOCBP) must not be pregnant or breastfeeding during any portion of the study and must use an adequate method to avoid pregnancy during the study period and for 6 months after treatment conclusion and agrees not to donate eggs (ova, oocytes) for the purpose of reproduction during the study and for a period of 6 months after last dose of study treatment. The Investigator should evaluate the effectiveness of the contraceptive method in relationship to the first dose of study treatment (see the Approved Methods of birth control listed below). * In order for a woman to be determined not of childbearing potential, she must have ≥ 12 months of non-therapy-induced amenorrhea or be surgically or medically sterile. * WOCBP must have a negative serum pregnancy test result at Screening and a negative urine pregnancy test result at the Baseline visit prior to the first dose of study treatment. * The Investigator is responsible for review of medical history, menstrual history, and recent sexual activity to decrease the risk for inclusion of a woman with an early undetected pregnancy. 10. Male participants are eligible to participate if they agree to the following during the treatment period and for at least 90 days after the last dose of study treatment: * Refrain from donating sperm PLUS either: * Be abstinent from heterosexual intercourse as their preferred and usual lifestyle (abstinent on a long term and persistent basis) and agree to remain abstinent until 90 days after the last study drug treatment; OR * Must agree to use a male condom when having sexual intercourse with a WOCBP and their female partner must utilize one the approved methods of birth control below: Approved Methods of birth control for this study are: * Total abstinence * Male or female sterilization (vasectomy in males or surgical removal of ovaries or uterus in females) * Unsterilized male study participants must use a male condom and their female partner must use one of the methods below: * Unsterilized female study participants must use one of the following highly effective methods listed below: Acceptable birth control methods which are considered highly effective if they result in a failure rate of less than 1% per year when used consistently and correctly: * Combined (estrogen and progestogen containing) hormonal contraceptive that stops the release of eggs from the ovary (oral, intravaginal, or transdermal) * Progestogen-only hormonal contraception that stops the release of eggs from the ovary (oral, injectable, implantable) * Intrauterine device (IUD) * Intrauterine hormone-releasing system (IUS) * Bilateral tubal occlusion or bilateral tubal ligation Exclusion Criteria: 1. Participant has a altered screening values: 1. alanine transaminase (ALT) value of \> 2.0 x upper limit of normal (ULN); 2. total bilirubin value of \> 1.5 x ULN (isolated bilirubin \> 1.5 x ULN is acceptable if bilirubin is fractionated and direct bilirubin \< 35%); 2. Participant has a history of malignancy associated hypercalcemia; 3. Participant has an active parathyroid disorder, hyperphosphatemia at Screening (serum phosphorus \> 1 x ULN), or serum calcium (mg/dL) x serum phosphorus (mg/dL) product \> 70 at Screening; 4. Any clinically significant active or known history of liver disease, or known hepatic or biliary abnormalities (with the exception of Gilbert's syndrome or asymptomatic gallstones); 5. Hepatitis serology will be tested at Screening. Participants who are hepatitis B surface antigen (HBsAg) positive or hepatitis C virus (HCV) antibody positive at Screening must not be enrolled until further definite testing with hepatitis B virus (HBV) deoxyribonucleic acid (DNA) titers is \< 500 IU/mL or HCV ribonucleic acid (RNA) polymerase chain reaction test is negative; 6. Lymphoma, leukemia, or any malignancy (including malignant glioma or MPNST) within the past 5 years except for basal cell or squamous epithelial carcinomas of the skin that have been resected with no evidence of metastatic disease for 3 years; 7. Breast cancer within the past 3 years; 8. Active optic glioma or other low-grade glioma requiring treatment with chemotherapy or radiation therapy