RecruitingPhase 1Drug trialA Study of SX-682 in Combination With Proton Craniospinal Irradiation (pCSI) in People With Leptomeningeal DiseaseThe researchers are doing this study to find out whether SX-682 in combination with proton craniospinal irradiation (pCSI) is a safe treatment approach for people with melanoma or non-small cell lung cancer (NSCLC) with leptomeningeal disease (LMD). The researchers will also look at whether the treatment combination is effective against participants' cancer.