ASPG and spondylo-ocular syndrome: Previously, SOS occurred extremely rarely during 6-MP therapy100 but recently has been described as a frequent complication to continuous polyethylene glycol-linkedEscherichia coli asparaginase preparation (PEG-asparaginase) during 6-MP-based maintenance therapy when combined with pulses of either HD-MTX or vincristine/dexamethasone, probably reflecting the impact of asparaginase on 6-MP pharmacokinetics causing higher drug metabolite levels104.