Renin inhibitor: relationship between molecular structure and oral absorption

New renin inhibitors with pseudodipeptidic units in P(1)-P(1') and P(2')-P(3') positions

A series of four new potential renin inhibitors has been synthesized. The structure of the compounds was designed in such a way as to produce agents resistant to enzymatic degradation, metabolically stable, possibly potent and with improved oral absorption. All positions of the 8-13 fragment of the human angiotensinogen were occupied by unnatural units (two unnatural amino acids in positions P(3) and P(2) and two pseudodipeptides in positions P(1)-P(1') and P(2')-P(3')). Both N- and C-terminal functions of the inhibitors were blocked with tert-Boc and ethyl ester groups. Their hydrophobicity evaluated as a log P value, calculated by a computer method, was 6.57 and 6.08 respectively. All peptides were obtained by the carbodiimide method in solution and purified by chromatography on the SiO(2) column. Their resistance to enzymatic degradation was assayed by determination of stability against chymotrypsin activity. The potency was measured in vitro by a spectrofluorimetric method (assay of Leu-Val-Tyr-Ser released from the N-acetyltetradecapeptide substrate by renin in the presence of the inhibitor). All inhibitors were stable to chymotrypsin. Their IC(50) (M/l) values were: 9.6 x 10(-4) (12), 1.6 x 10(-5) (17), 1.0 x 10(-5) (22) and 1.0 x 10(-5) (23) respectively.

Development of anti-influenza virus drugs I: improvement of oral absorption and in vivo anti-influenza activity of Stachyflin and its derivatives

PURPOSE

Stachyflin and its derivatives which are active against the influenza virus in vitro, were studied to improve their reduced in vivo activity after oral administration by chemical modification and some vehicles.

METHODS

The solubility was examined for different vehicles. The improvement of gastrointestinal absorption was evaluated by the plasma concentration after oral administration to mice or the in situ loop method with rats. The in vivo anti-influenza activity was examined using mice infected with the influenza virus and evaluated based on the virus titer in the lung by TCID50.

RESULTS

PEG 400 showed the highest solubility of Stachyflin and its derivative among the vehicles studied. While no viral inhibition was found in the lung after oral administration of 0.5% HPMC suspension of Stachyflin, in vivo anti-influenza virus activity was found with the PEG 400 solution. The absorption of Stachyflin by PEG 400 showed about a fifty-fold increase in AUC compared with that of 0.5% HPMC suspension. Improving the oral absorption of Stachyflin led to an increase in the in vivo anti-influenza virus activity. When the Stachyflin derivative in PEG 4000 was administered orally, there was more enhancement of the oral absorption than with PEG 400. When the aqueous solution of the phosphate ester prodrugs of Stachyflin and its derivative was administered orally, the absorption of the parent compound was improved and in vivo anti-influenza virus activity was found.

CONCLUSIONS

When Stachyflin and its derivatives were administered orally to mice with a solution in PEG and an aqueous solution of their phosphate ester, their oral absorption was improved and in vivo anti-influenza virus activity was observed.

Microencapsulation of the renin inhibitor FK906 by phase separation of ethylcellulose in cyclohexane

Microencapsulation of the renin inhibitor FK906 (tripeptide) by phase separation of ethylcellulose in cyclohexane was performed to obtain sustained release of the drug for a once-a-day application. Owing to the binding characteristics and to the very low solubility of FK906 in cyclohexane, microencapsulation can be performed after granulation of the drug with an inert filler, and no additional binder is required. Microcapsules with a particle size of 180-590 microns are obtained in a yield of 70%. Drug content determinations and SEM-micrographs reveal the almost complete incorporation of the polymer for the coating and the high quality of the microcapsule wall. Despite the strongly pH-dependent solubility of FK906 (.HCl) in water, the microcapsules show almost identical sustained-release curves at pH 1.2 and 6.0 (0.05 M phosphate buffer). This is explained by an acidic microenvironment inside the microcapsules at both pHs investigated and was attributed to the intrinsic physico-chemical properties of FK906 which help to overcome the buffer capacity of the phosphate buffer, pH 6.0, inside the microcapsules. This theory was confirmed by solubility experiments at pH 6.0 using excess amounts of FK906 as well as by dissolution tests as a function of the buffer capacity and the osmolality of the dissolution medium. The buffer capacity was found to be the parameter with greater influence on the release rate.

Transmembrane transport of peptide type compounds: prospects for oral delivery

Synthesis and delivery of potential therapeutic peptides and peptidomimetic compounds has been the focus of intense research over the last 10 years. While it is widely recognized that numerous limitations apply to oral delivery of peptides, some of the limiting factors have been addressed and their mechanisms elucidated, which has lead to promising strategies. This article will briefly summarize the challenges, results and current approaches of oral peptide delivery and give some insight on future strategies. The barriers determining peptide bioavailability after oral administration are intestinal membrane permability, size limitations, intestinal and hepatic metabolism and in some cases solubility limitations. Poor membrane permeabilities of hydrophilic peptides might be overcome by structurally modifying the compounds, thus increasing their membrane partition characteristics and/or their affinity to carrier proteins. Another approach is the site-specific delivery of the peptide to the most permeable parts of the intestine. The current view on size limitation for oral drug delivery has neglected partition considerations. Recent studies suggest that compounds with a molecular weight up to 4000 might be significantly absorbed, assuming appropriate partition behavior and stability. Metabolism, probably the most significant factor in the absorption fate of peptides, might be controlled by coadministration of competitive enzyme inhibitors, structural modifications and administration of the compound as a well absorbed prodrug that is converted into the therapeutically active agent after its absorption. For some peptides poor solubility might present a limitation to oral absorption, an issue that has been addressed by mechanistically defining and therefore improving formulation parameters. Effective oral peptide delivery requires further development in understanding these complex mechanisms in order to maximize the therapeutic potential of this class of compounds.

Chemical approaches to improve the oral bioavailability of peptidergic molecules

This review discusses both tools and strategies that may be employed as approaches towards the pursuit of orally active compounds from peptidergic molecules. Besides providing a review of these subjects, this paper provides an example of how these were utilized in a research programme at SmithKline Beecham involving the development of orally active GPIIb/IIIa antagonists. The tools for studying oral drug absorption in-vitro include variants of the Ussing chamber which utilize either intestinal tissues or cultured epithelial cells that permit the measurement of intestinal permeability. Example absorption studies that are described are mannitol, cephalexin, the growth hormone-releasing peptide SK&F 110679 and two GPIIb/IIIa antagonist peptides SK&F 106760 and SK&F 107260. With the exception of cephalexin, these compounds cross the intestine by passive paracellular diffusion. Cephalexin, on the other hand, crosses the intestine via the oligopeptide transporter. Structure-transport studies are reviewed for this transporter. The tools for studying oral drug absorption in-vivo involve animals bearing in-dwelling intestinal or portal vein catheters. A study of the segmental absorption of SK&F 106760 is provided. The review concludes with two chemical strategies that may be taken towards the enhancement of oral bioavailability of peptidergic molecules. The first strategy involves the chemical modification of peptides which enhance intestinal permeability, specifically the modification of amide bonds. The second strategy involves the design of compounds bearing nonpeptide templates, which are more amenable to the discovery of compounds with oral activity, from peptide pharmacophore models. An example is given regarding the discovery of SB 208651, a potent orally active GPIIb/IIIa antagonist, designed from the peptides SK&F 106760 and SK&F 107260.

Renin inhibitor: transport mechanism in rat small intestinal brush-border membrane vesicles

The transport characteristics of the renin inhibitor ((3S,4S)-4-[N-morpholinoacetyl-(1-naphthyl)-L-alanyl-N-methyl-(4-t hiazolyl)-L- alanyl]amino-3-hydroxy-5-cyclohexyl-1-(4-pyridyl)-1-pentanone; CH3-18) in rat small intestinal brush-border membrane vesicles (BBMV) were examined by a rapid filtration technique. The uptake of CH3-18 was markedly stimulated by an inwardly directed H+ gradient (pH 7.5 inside, pH 5.5 outside) and showed an uphill transport. It was competitively inhibited by tripeptides and tetrapeptides, but not by amino acids or dipeptides. A countertransport effect on the uptake of CH3-18 was observed in the vesicle preloaded with a tripeptide. Effects of the fragments of several renin inhibitors were evaluated by their inhibitory and countertransport effects on the uptake of CH3-18. The morpholino group at the N-terminal was found to be important for the uptake of CH3-18.