A novel mechanism for oral controlled release of drugs by continuous degradation of a phospholipid prodrug along the intestine: In-vivo and in-vitro evaluation of an indomethacin–lecithin conjugate
Introduction
Oral controlled release formulations provide a variety of pharmacokinetic and pharmacodynamic advantages [1]. Most of the available techniques to control the rate of drug release within the gastrointestinal tract (GIT) are formulation dependent and are based on the dissolution or diffusion rate of the drug from the dosage form [2]. There are only a limited number of methods where the rate of drug release is governed by endogenous enzymes in the intestine, such as colonic drug delivery systems that are based on degradation of a prodrug by microorganisms of the colon flora [3], [4] and colonic enzymes [5], [6]. The current investigation proposes a conjugate of an active compound that releases the drug moiety along the GIT due to degradation by endogenous phospholipase enzymes. Hence, the controlled release profile is not governed by the dosage form or any additional excipients, and may be achieved even when a liquid dosage form is used.
The prodrug approach, in which a derivative of the active compound is synthesized, has been proven to be an effective and important way of overcoming various ADME barriers that restrict the application of many chemical entities as orally administered drugs [7], [8].
Lipidic prodrugs are chemical entities comprised of two distinct parts: (1) the drug, which is covalently bound to (2) the lipid moiety. Three main different lipid carriers have been introduced: fatty acids, triglycerides and phospholipids. In the case of fatty acid, the drug is attached directly to the carboxylate or to the ω-position of the fatty chain. Triglyceride analogues have been prepared by substitution of one or more fatty acids of the triglyceride by a drug molecule. These approaches have been utilized on few nonsteroidal antiinflammatory drugs including aspirin [9], indomethacin [10] or naproxen [11], as well as other drugs including l-dopa [12], phenytoin [13] and GABA [14], in order to improve oral bioavailability, to improve brain blood barrier penetration and to reduce toxicity [15].
Use of drug–phospholipid conjugates has been suggested when the drug is bound to the phosphate group. This approach was utilized for cytostatic nucleosides and nucleoside analogues such as AZT [16], acyclovir [17] and cytidine analogues [18]. Phospholipids bearing a drug molecule instead of a fatty acid have not yet been investigated [19].
DP-155 is a novel prodrug of indomethacin chemically bound to phospholipid. The drug replaces a fatty acid at the sn-2 position of the phospholipid through a 5-carbon length linker (Fig. 1).
The purpose of the present study was to evaluate the liberation and absorption of free indomethacin following intravenous and oral administration of DP-155, and the corresponding pharmacokinetics, in order to evaluate the ability of DP-155 to serve as an indomethacin prodrug. In particular, we investigated the concept of continuous cleavage of DP-155 complex throughout the GIT by phospholipase A2 (PLA2) enzymes, whereby a controlled release of the active free drug along the GIT is achieved. An additional investigated parameter was the impact of the linker length between the phospholipid and the drug moiety on the rate and extent of the prodrug enzymatic degradation.
Section snippets
Materials
DP-155, DP-156, DP-157 and DP-158 were supplied by D-Pharm LTD (Rehovot, Israel). Indomethacin, ibuprofen, formic acid, ammonium acetate, Tris–HCl, NaCl, CaCl2 and propylene glycol were purchased from Sigma Chemical Co. (St. Louis, MO). Saline was obtained from Teva Medical (Ashdod, Israel). Ethanol, methanol, acetonitrile, water and ethyl acetate (J.T. Baker, Deventer, Holland) were HPLC grade. All other chemicals were of analytical reagent grade.
Bioavailability studies in rats
All surgical and experimental procedures were
Plasma profiles following intravenous administration
The plasma concentration versus time profiles of DP-155 and indomethacin following intravenous administration of DP-155 are shown in Fig. 2. Indomethacin plasma profiles following oral and intravenous administration are shown in Fig. 3, and the corresponding pharmacokinetic parameters are summarized in Table 1. Free indomethacin was formed in the plasma as a metabolite of DP-155 after intravenous administration. It can be seen that different indomethacin terminal slopes were obtained following
Pharmacokinetics
Comparison of the linear terminal slope of free indomethacin following intravenous administration and oral administration of DP-155 revealed that the decline of indomethacin in the body following its liberation from an oral DP-155 dose corresponds to the liberation half life of the drug from the prodrug conjugate, and not to the elimination half life of the drug from the body. This flip-flop kinetics indicates that the rate of drug input into the systemic circulation is slower than the kinetics
Conclusions
In this paper, we have introduced a novel mechanism of oral controlled release prodrug. The phospholipid–drug conjugate has been shown to release the free drug into the GI tract following degradation by PLA2 in a rate limiting manner. Thus, a controlled release profile of the free drug in the systemic blood circulation was obtained. The degradation of the drug–lecithin conjugate by PLA2 was found to be highly dependent on the length of the linker between the drug and the phospholipid.
This study
Acknowledgments
This work is a part of Arik Dahan PhD dissertation. This study was supported by the Israeli Consortium of Pharmalogica. The authors thank Dr. Josh Backon for the constructive comments. A. Hoffman is affiliated with David R. Bloom Center of Pharmacy.
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