Editor, – The authors of the article 'Pharmaceutical excipients – where do we begin?' (Aust Prescr 2011;34:112-4) make a very important point regarding the role of excipients in medications. Nowhere is this more relevant than in the treatment of epilepsy.
This concept has major ramifications for the use of generic drugs, but most recently we came across a series of patients who actually had significantly elevated blood concentrations of lamotrigine while remaining on the parent compound.1 Our initial worry was that these patients had been changed to a generic, but review of medication excluded that. Nothing in the way of measurement of their concentrations had changed. The pharmaceutical company producing the parent compound confirmed that they had sourced their product from a different manufacturing site. Consequently, the only plausible interpretation of the altered concentrations is that the excipient was altered, resulting in patients having altered bioavailability and hence marked increases in lamotrigine concentrations. Some patients experienced considerable toxicity.
The role of the excipient should not be underestimated and there is good reason to follow blood concentrations, particularly of antiepileptic medications, in patients who may be swapped from parent compound to generic. However, even the parent compound may equate to the equivalent of a generic if sourced from a different manufacturing site with possible different excipient.
Roy G Beran
Neurologist
Chatswood, NSW
