Potential interference of cyclobenzaprine and norcyclobenzaprine with HPLC measurement of amitriptyline and nortriptyline: resolution by GC-MS analysis

Cyclobenzaprine and its major metabolite, norcyclobenzaprine, differ from amitriptyline and nortriptyline only by the presence of a double bond in the cycloheptane ring. Three patients developed sufficient levels of cyclobenzaprine and norcyclobenzaprine because of either rapid or long-term ingestion of cyclobenzaprine to cause positive interferences in both a Syva EMIT assay and a high-performance liquid chromatographic (HPLC) assay for identification and quantitation of tricyclic antidepressants in serum. Cyclobenzaprine coeluted with amitriptyline, and norcyclobenzaprine eluted slightly earlier than, but was poorly resolved from, nortriptyline in this HPLC assay. We found that cyclobenzaprine could be distinguished from amitriptyline and that norcyclobenzaprine could be distinguished from nortriptyline on the basis of gas chromatographic retention times upon gas chromatographic-mass spectrometric analyses after derivatization with trifluoroacetic anhydride. The compounds were also distinguishable by mass spectrometric criteria.