The central hypothesis of the dissertation is that “the design and synthesis of a codrug of an opiate and a cannabinoid can be achieved which is stable in the gastrointestinal tract and shows a superior pharmacological and pharmacokinetic profile when compared to a physical mixture of the two parent drugs.” To prove the hypothesis, a series of novel codrugs were prepared by conjugation of the opiate drug codeine with Δ9-tetrahydrocannabinol (Δ9-THC), cannabidiol, abn-cannabidiol and an opiate prodrug 3-O-acetylmorphine with Δ9-THC. Codeine-cannabinoid codrugs were evaluated for analgesic activity in the rat after oral administration. The Cod-THC codrug showed greater effectiveness as well as prolonged pain management properties as compared to the parent drugs. The stability of Cod-THC codrug in aqueous solutions from pH 1-9, in simulated gastrointestinal fluids, in brain homogenate and the hydrolysis of the carbonate ester linkage in rat plasma suggested that after oral administration, the codrug would be absorbed intact from the GI tract and then hydrolyze in the plasma to generate both parent drugs. The enzymes present in rat brain homogenate were incapable of cleaving the codrug into the parent drugs. The pharmacokinetic profiles of the Cod-THC codrug and an equimolar physical mixture of the parent drugs were evaluated in rats. The plasma concentrations of codeine and Δ9-THC were much higher after codrug administration compared to the plasma concentrations of these drugs after oral administration of an equimolar physical mixture. The parent drugs were also present in the plasma for longer period of time compared to the physical mixture, probably due to the sustained release of the parent drugs from codrug in the plasma. The concentrations of codeine and Δ9-THC were much higher in rat brain after oral administration of the Cod-THC codrug as compared to brain concentrations of these drugs after oral administration of the physical mixture. Thus, the design and synthesis of an opiate and a cannabinoid codrug was achieved which was stable in the gastrointestinal tract, showed enhanced analgesic effects as compared to the parent drugs, and also showed a superior pharmacokinetic profile when compared to a physical mixture or the two parent drugs.

Pain is an unpleasant sensory and emotional experience associated with actual or potential tissus damage or described in terms of such damage. Opioids are effective in treating moderate to severe pain, but opioid alone therapy is associated with several adverse effects, development of tolerance and addiction potential. One way to solve these problems is to administer opioids with adjuvant drugs. In this project several opioid molecules were combined with other adjuvant drugs in a single chemical entity to form a codrug. A series of codrugs were prepared by conjugation of an opioid with S-(-)-nornicotine, ketamine, norketamine and gabapentin. Several of the synthesized codrugs were evaluated for analgesic activity in the rats after oral administration. Codeine-S-(-)- nornicotine, 3-O-acetylmorphine-S-(-)-nornicotine, and N-ethoxycarbonylgabapentincodeine codrugs showed greater effectiveness as well as prolonged pain management properties as compared to the parent drugs. Stabilities of several synthesized codrugs were studied in aqueous solutions from pH 1.3-7.4, in simulated gastrointestinal fluids, in rat plasma and in brain homogenate. Only the ester-linked codrugs showed sign of hydrolysis in different solutions. Carbamate-linked codrugs didn’t cleave under any hydrolytic condition. Pharmacokinetic study was performed on the following three codrugs: 3-O-acetylmorphine-S-(-)-nornicotine, N-acetylgabapentin-codeine, and N-ethoxycarbonylgabapentin- codeine. The carbamate linkage in 3-O-acetylmorphine-S-(-)- nornicotine codrug did not cleave in vivo to produce parent drugs. The ester linkage in N-acetylgabapentin- codeine codrug cleaved in vivo to produce codeine and N-acetylgabapentin, but N-acetylgabapentin did not undergo hydrolysis to produce gabapentin. The ester linkage in N-ethoxycarbonylgabapentin-codeine codrug hydrolyzed slowly in plasma to produce N-ethoxycarbonylgabapentin and codeine and then the carbamate linkage in N-ethoxycarbonylgabapentin hydrolyzed even slowly to produce gabapentin. Produced codeine also metabolized to generate some amount of morphine. Thus, the design and synthesis of an opiate and gabapentin codrug was achieved which was stable enough in the gastrointestinal tract, showed enhanced analgesic effects as compared to the physical mixture of the parent drugs, and also produced the two parent drugs in blood plasma.