This is a single-arm, open-label, multicenter, exploratory study initiated by investigator to evaluate the efficacy and safety of Selinexor (S) combined with pomalidomide (P) and dexamethasone (D) in the treatment of multiple myeloma with CNS involvement.

This is a single-arm, open-label, multicenter, exploratory study. The study plans to enroll patients diagnosed with multiple myeloma with CNS involvement to receive Selinexor (60 mg QW), pomalidomide (4 mg/day, on Days 1-21), and dexamethasone (40 mg/day, on Days 1, 8, 15, 22), with all agents dosed 28 days per cycle.

If patients reach maximum efficacy and have received SPD treatment for 8-12 cycles, maintenance treatment of Selinexor (20-60mg QW, optimal dose determined by the investigator) will be started and continued until disease progression or intolerable adverse events.

Inclusion Criteria:

1. Age ≥ 18 years.
2. Patients with multiple myeloma with measurable lesions as assessed using the updated International Myeloma Working Group Response Criteria.
3. Multiple myeloma patients with central nervous system involvement (CNS-MM): the presence of plasma cells in the cerebrospinal fluid (CSF), and/or leptomeningeal, dural, or intraparenchymal involvement as assessed using imaging modalities (MRI, CT or PET-CT), and/or tissue biopsy; or eye involvement confirmed by ophthalmologists, with or without neurological symptoms.
4. ECOG ≤ 2, with expected survival of more than 3 months.
5. No active infectious diseases.
6. Major organs without serious organic diseases (except renal insufficiency caused by multiple myeloma).
7. Female patients are not during pregnancy or breastfeeding; fertile men or women agree to use appropriate methods of contraception 4 weeks before study treatment, during treatment and drug suspension, and within 4 weeks after the end of study treatment.
8. Be able to cooperate with study follow-up. Patients understand the disease characteristics and voluntarily join this study to receive treatments and study follow-ups.
9. Have signed informed consent. Informed consent was signed by the patient or by an immediate family member. Considering the patient 's condition, if the patient' s signature is unfavorable for disease treatment, the informed consent form should be signed by the legal guardian or the patient 's immediate family member.

Exclusion Criteria:

1. Patients with active hepatitis B (HBV), hepatitis C (HCV), and other acquired, congenital immunodeficiency diseases;
2. According to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE) 5.0, baseline with grade 2 or higher peripheral neuropathy or neuralgia;
3. Severe thrombotic events before treatment;
4. Patients with uncontrolled or severe cardiovascular disease, including myocardial infarction within 6 months before enrollment, New York Heart Association (NYHA) defined grade III-IV heart failure, uncontrolled angina pectoris, clinically significant pericardial disease or cardiac amyloidosis;
5. Major surgery within 30 days before enrollment;
6. Epilepsy and dementia which requires drug treatment, and other mental disorders which cannot understand or follow the study protocol;
7. According to the protocol or the investigator 's judgment, any severe physical illness or mental illness may interfere with participation in this clinical study;
8. Drug abuse, medical, psychological or social conditions that may interfere with the participation of subjects in the study or evaluation of study results;
9. Patients who are receiving other experimental treatment;
10. Pregnant or lactating women;
11. Not suitable for enrollment per investigators' judgements.