Intrathecal Pemetrexed for SCLC Patients With Refractory Brain Metastases or Leptomeninges Metastatic

Inclusion Criteria: * 1\. Understand the requirements and contents of the clinical trial, and provide a signed and dated informed consent form. * 2\. Age ≥ 18 years. * 3\. Histopathology is confirmed small cell lung cancer . * 4.Leptomeningeal metastasis (LM) is defined by the presence of typical clinical symptoms, positive cerebrospinal fluid (CSF) cytology, detection of cell-free DNA (cfDNA) in CSF by next-generation sequencing (NGS), or imaging findings consistent with typical meningeal metastases. * 5.Patients with brain progression after whole-brain radiotherapy. * 6\. Predicted survival ≥ 12 weeks. . * 7\. ECOG 0-2. * 8\. Adequate bone marrow hematopoiesis and organ function. Exclusion Criteria: * 1\. Previously received intrathecal pemetrexed therapy for locally advanced or metastatic disease. * 2\. Subjects who have received any of the following treatments must be excluded: * Have received radiation within 14 days prior to the first dose or have not recovered from radiation-related toxicity. Chest and extra-brain palliative radiotherapy, stereotactic radiosurgery, and stereotactic body radiotherapy may be performed 7 days prior to the first dose. * 3\. Presence of spinal cord compression or meningeal metastasis. * 4\. History of other malignant tumors within 2 years. * 5\. Adverse events (except alopecia of any degree) of CTCAE \> grade 1 due to prior treatment (e.g., adjuvant chemotherapy, radiotherapy, etc.) prior to the first dose. * 6\. History of stroke or int…