Compositions and methods for increasing the metabolic lifetime of dextromethorphan and related pharmacodynamic effects

1. A method of enhancing dextromethorphan plasma levels in a human being, comprising co-administering a bupropion with a dextromethorphan twice a day, to the human being, for at least 14 consecutive days, wherein the human being is an extensive metabolizer of dextromethorphan in need of treatment with dextromethorphan, wherein the bupropion and the dextromethorphan, are the sole active agents administered to the human being, wherein the bupropion is administered in an amount that, on the eighth day that the bupropion and the dextromethorphan are co-administered twice a day, results in: 1) a C max of dextromethorphan that is at least about 40 ng/mL, 2) an AUC 0-12 of dextromethorphan that is at least about 450 ng·hr/mL, 3) a C max of bupropion that is at least about 50 ng/mL, 4) a C max of hydroxybupropion that is at least about 300 ng/mL, 5) a C max of erythrohydroxybupropion that is at least about 30 ng/mL, and 6) a C max of threohydroxybupropion that is at least about 100 ng/mL, and wherein, on the eighth day that the bupropion and the dextromethorphan are co-administered twice a day, the C max of dextromethorphan is at least about 15 times the C max of dextromethorphan that would result from administering the same amount of the dextromethorphan without the bupropion twice a day for eight consecutive days.

2. The method of claim 1 , wherein the bupropion and the dextromethorphan are orally administered.

3. The method of claim 2 , wherein the bupropion and the dextromethorphan are orally administered in a single, solid dosage form.

4. The method of claim 3 , wherein the single, solid dosage form further comprises a binder, an excipient, a disintegrating agent, a lubricant, a sweetening agent, a flavoring agent, a coating, or a capsule.

5. The method of claim 1 , wherein the bupropion is in a form that contains an enantiomeric excess of the S-enantiomer.

6. The method of claim 1 , wherein the bupropion is in a form that contains an enantiomeric excess of the R-enantiomer.

7. The method of claim 1 , wherein the bupropion is deuterium-modified.

8. The method of claim 1 , wherein the dextromethorphan is deuterium-modified.

9. The method of claim 1 , wherein the bupropion is administered in a manner that maximizes dextromethorphan plasma levels in the human being.

10. The method of claim 1 , wherein the C max of dextromethorphan on the eighth day is at least about 45 ng/mL.

11. The method of claim 1 , wherein the C max of dextromethorphan on the eighth day is at least about 55 ng/mL.

12. The method of claim 1 , wherein the C max of dextromethorphan on the eighth day is at least about 75 ng/mL.

13. The method of claim 1 , wherein the C max of dextromethorphan on the eighth day is at least about 30 times the C max of dextromethorphan that would result from administering the same amount of the dextromethorphan without the bupropion twice a day for eight consecutive days.

14. The method of claim 1 , wherein the C min of dextromethorphan on the eighth day is at least about 25 ng/mL.

15. The method of claim 1 , wherein about 44 mg of the dextromethorphan is administered to the human being twice a day.

16. The method of claim 1 , wherein about 45 mg of the dextromethorphan is administered to the human being twice a day.

17. The method of claim 1 , wherein about 46 mg of the dextromethorphan is administered to the human being twice a day.

18. The method of claim 1 , wherein about 44 mg to about 46 mg of the dextromethorphan is administered to the human being twice a day and about 100 mg to about 110 mg of the bupropion is administered to the human being twice a day.

19. The method of claim 18 , wherein the bupropion is co-administered with the dextromethorphan for at least 30 consecutive days.

20. The method of claim 1 , wherein less than 300 mg of bupropion is administered per day in order to reduce the risk of seizure.