| Affecting Intestinal First Pass Metabolism Processes of Orally Administered Drugs by Pharmaceutical Methodology: The Cannabinoids Paradigm
Low solubility, first pass metabolism process and efflux by P-glycoprotein (P-gp) pumps are commonly accounted for extremely low and highly variable oral bio-availability of poor water soluble compounds. Traditionally, liver was assumed to be the major metabolic organ responsible for the first-pass metabolism of an orally administered drug. Recently, the hypothesis that many drugs have poor oral bio-availability due to biological barriers at the enterocyte level, termed “intestinal first pass metabolism”, was raised. These biological processes include Phase I oxidative metabolism by CYP450 enzymes, efflux by P-gp pumps and Phase II metabolism. The classical view was that Phase II enzymes catalyze the conjugation processes of metabolites formed by Phase I CYP enzymes. However, over the last decade, evidence of a direct glucuronidation of parent drugs mainly by UDP-glucuronosyltransferases (UGTs) in the enterocytes is starting to be accumulated. This way Phase II metabolism can significantly contribute to decreased oral bioavailability. While there are several methods to inhibit metabolic and efflux processes by pharmacological agents, there are limited pharmaceutical solutions to overcoming these processes. Much more limited is the case of Phase II metabolism, where the knowledge is insufficient and currently there are barely any available pharmaceutical technologies in the clinic that may control this absorption barrier.
Several alkaloids and polyphenolic compounds, often used as food supplements, were reported as modifying the pharmacokinetics (i.e. absorption and disposition) of drugs by multiple mechanisms; inhibition of P-gp efflux, inhibition of CYP enzymes and reduction of phase II metabolism by inhibition of UGTs. Thus, many studies define these compounds as “absorption enhancers”. However, their utilization in enhancing the oral bio-availability of drugs is very limited due to their poor water solubility that prevents their presence at appropriate concentrations required to saturate these absorption barriers.
The general goal of this research is to investigate biopharmaceutical approaches which can be utilized to improve the oral bio-availability of drugs subjected to extensive intestinal first pass effect due to efflux pumps activity, Phase I and Phase II metabolism. This will be achieved by the utilization of a unique lipid based drug delivery system termed Pro-Nano-Lipospheres (PNLs) as well as PNLs incorporating an absorption enhancer, termed Advanced-PNLs, as research tools.
PNLs are isotropic homogenous mixtures of an active compound in a combination of lipids, surfactants and co-solvents termed pre-concentrates. Upon gentle agitation in an aqueous phase, such as the upper GI lumen content, these pre-concentrates spontaneously form drug encapsulated O/W nano-emulsions with a particle diameter of 100 nm or less. The prevailing view was that PNL enhances the oral bioavailability of poorly water soluble drugs by increasing their solubility. A previous study conducted by our group indicated that additional mechanisms may be involved, including inhibition of intestinal Phase I metabolism and P-gp efflux of the incorporated drug1. In the current work, we aim to further investigate the biopharmaceutical aspects of PNLs and to broaden their utilization in enhancing oral bioavailability by the addition of absorption enhancers to develop Advanced-PNLs. Our working hypothesis is that PNL can be used as a platform for the synchronized delivery of the absorption enhancer and the drug to the absorption site of the enterocyte. In this manner PNL will enable the simultaneous delivery of the poor water soluble absorption enhancer and the poorly water soluble drug to the enterocyte surface in their solubilized state. The investigation of the effect of absorption enhancers (delivered by A-PNL) on Phase II metabolism will provide a novel platform for enhancing the bio-availability of drugs subjected to extensive first pass phase II metabolism. Model molecules used to investigate the effect of PNLs and Advanced-PNLs on intestinal absorption barriers are poor water soluble and highly metabolized cannabinoids tetrahydrocannabinol (THC) and cannabidiol (CBD) and an exclusive Phase II substrate raloxifene.
To investigate the effect of PNL and absorption enhancers on the intestinal absorption barriers CBD-PNL, THC-PNL and 3 advanced CBD-PNLs incorporating curcumin, resveratrol or piperine were developed and characterized in-vitro. Additionally, the effect of PNL and an absorption enhancer piperine on exclusively Phase II metabolism was investigated by the co-administration of blank-PNL and piperine-PNL with raloxifene. Pharmacokinetic (PK) parameters were evaluated in pre-clinical studies using a freely moving rat model. The effect of Advanced-PNL incorporating piperine on the oral bio-availability of THC and CBD was investigated in clinical studies (on healthy human volunteers) using commercially available buccal spray Sativex® containing THC and CBD as a control.
The administration of single oral CBD-PNL to rats resulted in a 3-fold and a 3.5-fold increase in the AUC and Cmax, respectively as compared to CBD. The administration of single oral THC-PNL to rats resulted in a 6-fold and a 5-fold increase in the AUC and Cmax, respectively as compared to THC. Curcumin precipitated from PNL 10 min. following its introduction into the water phase allowed a sufficient time-window in which to perform in-vivo studies in rats. Incorporation of resveratrol and piperine formed homogeneous PNLs with particles of 65nm and 30nm in size, respectively. However, administration of these absorption enhancers in Advanced-PNL had a different effect on the oral bio-availability of CBD. The relative oral bio-availability of CBD-resveratrol-PNL following single oral administration to rats was not significantly different as compared to CBD-PNL. Single oral administration of CBD-piperine-PNL to rats resulted in a 2-fold increase in the AUC as compared to CBD-PNL. Single oral administration of THC-piperine-PNL to rats resulted in a 1.47-fold increase in the AUC as compared to THC-PNL. Our clinical studies corroborate the results of pre-clinical studies and indicate that the incorporation of THC and CBD into piperine -PNL significantly increases the AUC and Cmax values of THC and CBD as compared to Sativex®.
The pre-clinical and clinical study results indicate that our PNL is an effective platform for enhancing the oral bio-availability of poor water soluble and highly metabolized compounds such as THC and CBD. We have shown that upon incorporation of piperine into PNL an additional increase in the oral bio-availability of the two cannabinoids is achieved. This additional increase in the oral bio-availability is achieved due to the simultaneous delivery of the absorption enhancer and the drug by PNL to the enterocyte surface in their solubilized state2. Our pre-clinical PK studies indicate that the enhanced oral bio-availability of poorly water soluble drugs incorporated into PNL and Advanced-PNL is accomplished by the modification of an absorption phase by these delivery systems. As the incorporation of such drugs into PNL and Advanced-PNL does not alter their elimination phase, the enhanced oral bio-availability is attributed to the inhibition of intestinal rather than hepatic first-pass metabolism processes.
The Piperine-PNL biopharmaceutical methodology developed in this work is unique and can be effectively utilized to improve the oral bio-availability of direct intestinal Phase II substrates. This novel delivery system will enable the advanced development stages of Phase II substrates which would otherwise fail to proceed towards clinical use.
- Elgart A, Cherniakov I, Aldouby Y, Domb AJ, Hoffman A. Improved Oral Bioavailability of BCS Class 2 Compounds by Self Nano-Emulsifying Drug Delivery Systems (SNEDDS): The Underlying Mechanisms for Amiodarone and Talinolol. Pharm Res 2013 May;30(12):3029-44.
- Patent: CBD/THC oral formulations. Inventors: A. J. Domb, A. Hoffman, I. Cherniakov.