when was the first time that liposomal cytarabine (depocyt) was prescribed to patient 64538?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64538 ) and prescriptions.drug = 'liposomal cytarabine (depocyt)' order by prescriptions.startdate asc limit 1