Levo-tetrahydropalmatine (l-THP) is an active herbal constituent of preparations containing plant species of the genera Stephania and Corydalis. In China, it has been approved and used for a number of clinical indications under the drug name Rotundine.

In vitro: In contrast to other THPB derivatives, l-THP is an antagonist at both of D1 and D2 receptors. The Ki values for l-THP at D1 and D2 dopamine receptors are approximately 124 nM (D1) and 388 nM (D2), while the IC50 values are 166 nM and 1.4 μM for D1 and D2, respectively. The relatively high affinity of l-THP at D1 vs. D2 receptors, distinguishes it from other available dopamine receptor antagonist drugs (e.g., haloperidol) [1].

In vivo: It has been found that l-THP produces dose-dependent reductions in locomotor activity and operant responding for non-drug reinforcers in rats [1].

Clinical trials: It has long been recognized l-THP has therapeutic values for treating a number of CNS related conditions. The effectiveness of l-THP as a non-opioid analgesic agent resulted in the approval of purified l-THP for this indication by the Chinese SFDA and has led to extensive investigation of the pharmacological properties of l-THP [1]

Reference:
[1] Wang JB, Mantsch JR.?? l-tetrahydropalamatine: a potential new medication for the treatment of cocaine addiction. Future Med Chem. 2012 Feb;4(2):177-86.