what is the dose of sucralfate that is last prescribed to patient 53015 until 05/2105?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 53015 ) and prescriptions.drug = 'sucralfate' and strftime('%y-%m',prescriptions.startdate) <= '2105-05' order by prescriptions.startdate desc limit 1