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Clinical consequences of generic substitution of lamotrigine for patients with epilepsy

From the Centre de Recherche (J.L., O.S.), Centre Hospitalier de l'Université de Montréal; Analysis Group, Inc. (M.S.D., J.W.), Boston, MA; Groupe d'Analyse Ltée (P.E.P., P.L.), Montréal, Québec, Canada; and GlaxoSmithKline (R.M.), Research Triangle Park, NC.

Address correspondence and reprint requests to Dr. Mei Sheng Duh, Vice President, Analysis Group, Inc., 111 Huntington Avenue, Tenth Floor, Boston, MA 02199 mduh{at}analysisgroup.com

Objectives: To measure the proportions of patients switching from generic to branded drugs among users of antiepileptic drugs (AED) compared to other therapeutic areas and to investigate medical services utilization associated with generic switching of lamotrigine.

Methods: Medical and pharmacy claims data from Régie de l'Assurance Maladie du Québec database from April 1998 to July 2006 were used. Patients with an epilepsy diagnosis (International Classification of Diseases–9 345) and treated with lamotrigine for >60 of the 90 days before the entry date of generic lamotrigine in Quebec (February 1, 2003) were selected. The proportion of patients switching back to brand were calculated for lamotrigine, for other AEDs (clobazam, carbamazepine CR, gabapentin) and for non-AED chronic medications (carvedilol, fosinopril, simvastatin). Medical resource utilization was compared between periods of branded vs generic use of lamotrigine.

Results: Of 671 patients treated with branded lamotrigine, 187 patients (27.9%) switched to a generic, and 51 of these patients (27.5%) switched back to the branded medication. Rates of switchback were from 20.8% to 44.1% for various AEDs and from 7.7% to 9.1% for non-AEDs. Relative to the branded lamotrigine use period, generic lamotrigine use period was associated with a 5.1% increase in mean daily dose of lamotrigine (239.1 vs 251.4 mg; p = 0.0149), a higher number of dispensations for other AEDs (20.4 vs 23.9 dispensations per person-year; p < 0.001) as well as non-AED drugs (26.4 vs 32.8 dispensations per person-year; p < 0.0001), a higher utilization rate of medical services (8.7 vs 9.8 visits per person-year; p < 0.0001), and a longer hospital length of stay (3.29 days vs 4.86 days per person-year; p < 0.0001).

Conclusion: A higher propensity to switch back to branded medications was observed among antiepileptic drug users compared to users of antihypertensives and antihyperlipidemics, similar to findings from Andermann et al. Switch to generic lamotrigine was significantly associated with increased physician visits and hospitalizations.

GLOSSARY: AED = antiepileptic drugs; HR = hazard ratio; ICD = International Classification of Diseases; RAMQ = Régie de l'Assurance Maladie du Québec; RD = rate difference; RR = rate ratio.

Received May 16, 2007. Accepted in final form August 28, 2007.

Disclosure: This study was sponsored by GlaxoSmithKline (GSK), Research Triangle Park, NC. GSK participated in the design, review, and approval of the manuscript. J. LeLorier has a consulting agreement with Analysis Group, Inc. and has received grants from GSK for other research projects in excess of $10,000. M.S. Duh, P.E. Paradis, P. Lefebvre, and J. Weiner are employees of Analysis Group, Inc., which has received research grants from GSK. R. Manjunath is an employee of GSK. O. Sheehy reports no conflict of interest.

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