William J. Wechter

Drug chirality: on the mechanism of R-aryl propionic acid class NSAIDs. Epimerization in humans and the clinical implications for the use of racemates.

This review summarizes and comments on the current understanding of both the biochemical and clinical implications of the epimerization of R‐aryl propionic (APA) class (1) nonsteroidal anti‐inflammatory agents (NSAIDs) to S‐enantiomers in humans. This article focuses principally on rac‐ibuprofen and its enantiomers. In the United States, five commercialized NSAIDs are APAs. Only two of them, rac‐ibuprofen and rac‐fenoprofen, are subject to significant epimerization in humans. The remaining three, rac‐flurbiprofen, rac‐ketoprofen, and S‐naproxen, are not of interest in this context.

Synthesis of Aminobisphosphonate

Abstract A new facile synthesis of aminobisphosphonate was reported. Dibenzylamine bisphosphonate (1) is prepared from dibenzylamine, triethyl orthoformate and diethyl phosphite. Deprotection by hydrogen transfer reaction and acid hydrolysis afforded aminobisphosphonate (2).

From controversy to resolution: bioequivalency of racemic drugs--a symposium on the dynamics, kinetics, bioequivalency, and analytical aspects of stereochemistry.

T he symposium was organized by F. Jamali, (U. of Alberta) and co-chaired by W. J. Wechter, (Loma Linda University). We anticipate that Drug Chirality will become a continuing focus for the ACCP both in this Journal and at our National Meeting. The influence of chirality on pharmacology has been and continues to be debated. Many of the biologic conclusions have been speculative following from pharmacokinetic studies. Many pronouncements have been made that lack experimental validation. We expect that with the development of a general understanding of the issues there will be proper attention to chiral pharmacokinetics, chiral pharmacodynamics, and chiral purity. We hope that this symposium will help set standards for publication within this field. Prominent among the concerns of the pharmaceutical industry has been the impact of this emerging field of generic substitution of drugs as their patents die. This symposium was conceived in large part to help illuminate the scientific and regulatory questions that bear on issues of bioequivalency. To this end there were five major presentations, two of which, Galdwell and Pasutto, set the basic science stage for the debate. Jamali followed with a discussion of the scientific issues associated with bioequivalency and stereoselectivity. Regulatory views were presented by Williams (United States) and Nitchuk (Canada). These presentations are elaborated as full manuscripts in this issue of the Journal. The Selected Posters Session enlarged on some of these subjects. Abstracts of the posters also appear in this issue of the Journal. In addition we asked a few poster participants to submit full papers because of the novelty of their work.

Mechanism of Enhancement of Intestinal Ulcerogenicity of S-Aryl Propionic Acids by Their R-Enantiomers in the Rat

We previously observed a marked increase ingastrointestinal toxicity of rac -flurbiprofen comparedto the therapeutically equivalent dose of the Senantiomer. This paper quantitates these observations and examines the mechanism by which thisparadoxical toxicity occurs. We have evaluated the ulcerscores, mucosal neutrophil infiltration, byimmunostaining of CD11/18 antigen, and mucosalneutrophil activity by myeloperoxidase measurement at two doselevels of (R)-, (S)-, and rac-flurbiprofen, administeredover 30 days. Dose-response for intestinal ulcerproduction was observed for rac- and (S)-flurbiprofen; animals given (R)flurbiprofen exhibited noulcers. Yet rac-flurbiprofen proved to be twice asulcerogenic as (S)-flurbiprofen. The mechanism of theexacerbation of gastrointestinal toxicity of(S)flurbiprofen by the noncyclooxygenase inhibiting(R)-flurbiprofen is believed to be associated with itseffect on ICAM-1 up-regulation. This is followed byneutrophil adhesiveness to ICAM-1 via the LFA-1 antigenon its surface and the extravasation ofneutrophils into the tissue. We also examined the effectof high dose (R)-flurbiprofen vs vehicle over 15 days inanimals in which ulcers had been produced by treatment with (S)-flurbiprofen for the previous 15 days.(R)-flurbiprofen did not sustain induced ulcers. Theresults of this study suggest that human studies beconducted to determine if enhanced gastrointestinal toxicity occurs in man. This is at issue sincerac compounds of this class are available over thecounter and others may be introduced.

R-Flurbiprofen (E-7869), a chemopreventive and treatment of cancer

R-flurbiprofen, the non-COX inhibiting enantiomer of the non-steroidal antiinflammatory (NSAI) agent R-, S-flurbiprofen has proved to have a pharmacology independent of the S-enantiomer. In this presentation, we recount its anti-proliferative activity in the rat in addition to its anti-cancer activity without significant ulcerogenicity in the Min and TRAMP mouse models. In the Min mouse (a model of human FAP) R-flurbiprofen proved curative. It had significant activity against both the primary lesion in the TRAMP mouse (a model of prostate cancer) and metastatic disease which was influenced by the amount of saturated fat in the diet. ___TAGSTART___BR___TAGEND___Surprisingly, we found R-flurbiprofen anti-inflammatory in the rat hind-paw edema assay. From our accumulating data, and the work of others, R-flurbiprofen has pro-apoptotic activity. We hypothesize that the mechanism of R-flurbiprofens pro-apoptotic activity is inhibition of COX-2 m-RNA transcription and/or down-regulation of NFkB. We believe that R-flurbiprofen will delay or prevent progression of epithelial pre-malignant lesions to carcinoma (colon, prostate, lung and breast) without significant side effects. In addition, as hypothesized for other modifiers of cyclo-oxygenase activity, it may delay or prevent progression of Alzheimers Disease. Phase I and IIa clinical studies with oral R-flurbiprofen (E-7869) have begun in normal volunteers and prostate cancer patients.

Endogenous natriuretic factors 1: Sodium pump inhibition does not correlate with natriuretic or pressor activities from uremic urine

It is our purpose to isolate and characterize the putative Natriuretic Hormone, ostensibly responsible for ECF homeostasis, as well as identify endogenous pressors and compounds that induce prolonged natriuresis; we report here our initial progress in this area. Large volumes of pooled urine collected from uremic patients were fractionated, and the resulting isolates were evaluated for in vivo natriuretic and pressor effects and Na+/K(+)-ATPase inhibitory activity in renal cells. The purification steps involved ultrafiltration to obtain materials of less than 3000 da, gel filtration, and sequential reversed-phase high performance liquid chromatography (HPLC). After each HPLC step, the fractions were evaluated for their ability to elicit significant natriuresis and/or influence mean arterial pressure in the normal conscious female rat. Each fraction was also assayed for its ability to inhibit Na+/K(+)-ATPase as determined by the inhibition of 86Rb+ uptake into MDBK renal cells. While several of the fractions elicited profound natriuresis and/or pressor activity and other fractions inhibited Na+/K(+)-ATPase, there was no correlation among the activities in individual fractions. We have concluded that this plethora of bioactivities is responsible for much of the confusion and multiplicity of crude isolates claimed to be the putative hormone. Presently we are attempting to purify each of these activities to chemical homogeneity for structure determination.

Human Plasma Concentrations of R, S, and Racemic Flurbiprofen Given as a Toothpaste

Flurbiprofen, an arylpropionic acid (APA) class nonsteroidal antiinflammatory drug (NSAID), is commercially available only as the racemic mixture, although its pharmacologic effect has been credited primarily to the S isomer. In humans, the bioavailability of racemic flurbiprofen absorbed from the oral cavity has been studied measuring the total concentration of S‐ and R‐flurbiprofen, and the pharmacokinetics of S‐ and R‐flurbiprofen have been studied after oral administration of racemic flurbiprofen. In this study, the plasma concentrations of S‐flurbiprofen and to some extent R‐flurbiprofen were studied after brushing with a toothpaste containing different mixtures of S‐ and R‐flurbiprofen. The toothpaste formulations contained 1% racemic (50:50), eutectic (14:86), 1%, 0.5%, and 0.25% (5:95) R‐ to S‐flurbiprofen. Both S‐ and R‐flurbiprofen were rapidly absorbed, with a time to reach maximum concentration (tmax) of 1.2 to 1.4 hours. Based on the AUC, the amount of S‐flurbiprofen absorbed increased proportionally when given as the 0.25% (5:95) preparation to the 0.5% (5:95) mixture but did not increase significantly above the 0.5% (5:95) mixture when given as 1% (5:95) R‐ to S‐flurbiprofen. This suggests that dose‐proportional absorption of S‐flurbiprofen is not maintained at higher concentrations. The elimination of S‐flurbiprofen appears to be variable and prolonged after this mode of administration, as observed from plasma concentrations. Further controlled and more prolonged studies of S‐ and R‐flurbiprofen are needed to confirm these observations.

Calculation of Inversion Half‐Lives of Aryl Propionic Acid Class Nonsteroidal Antiinflammatory Drugs

Although inversion of the R‐enantiomers of the aryl propionic acid (APA) class of nonsteroidal antiinflammatory drugs (NSAIDs) in humans and other mammals has been known for more than 20 years, no satisfactory method has been developed for evaluating the half‐life of the inversion process. This parameter is useful in assessing the pharmacodynamic contribution of the R‐prodrug to the cyclooxygenase‐inhihiting S‐enantiomers. Further, it provides a similar means of evaluating the analgesic contribution of R‐enantiomers to racemic mixtures. Using human and animal data, we have mathematically determined the inversion half‐life (t1/2i) for ibuprofen, ketoprofen, and fenoprofen. The relation of the sensitivity of this value to the terminal half‐life (t1/2) of the R‐enantiomers of APA class drugs also is discussed.

Natriuretic hormones II

Renal sodium metabolism, an important determinant of blood pressure, is regulated with great precision by a variety of endocrine, autocrine, and neuronal factors. Although these factors regulate sodium metabolism by affecting the rate of tubular sodium reabsorption, the molecular mechanisms by which they act are poorly understood. It has been proposed that the antidiuretic agents angiotensin II and norepinephrine and the diuretic agents dopamine and atrial natriuretic peptide (ANP) may achieve their effects through a common pathway that involves a bidirectionally regulated intracellular protein phosphorylation cascade. This in turn modulates the activity of renal tubular Na+/K+-ATPase [1]. An exaggerated natriuretic response to acute volume expansion by intravenous saline has been demonstrated in essential [2] as well as borderline hypertension [3]. The overall response to acute volume expansion by intravenous saline has been characterized by a complex humoral pattern which includes the acute suppression of the renin-angiotensin-aldosterone system [4]. Recent studies by Bullivant et al. [5] demonstrate that plasma levels of angiotensin II, prostaglandins, kinins, dopamine, ANP, adrenal hormones, or factors released by the renal nerves are unchanged by renal arterial saline infusion. Thus none of these is directly responsible for the saline induced natriuresis.

Overview of the ACCP Symposium on Chiral Pharmacology, Washington, DC, September 23, 1995

Three of these presentations are included in this issue in their entirety, as are all of the abstracts. D. Triggle (SUNY, Buffalo)presented in his introductory talk a paradigm for the effect of receptors in the state dependent interaction ion channels. Ion channels are receptors with specific drug binding sites that define structure-function relationships. These state-dependent interactions determine affinity and access of drug to binding sites. This can lead to both quantitative and qualitative changes in structure-function relationships, including chiral selectivity. Examples cited were voltage gated Na and Ca+2 channels, possessing as many as five binding sites: GABAA-gated CIchannels possess a similar number. Stereoselectivity of drug action at these binding sites includes local anesthetics and antiarrhythmics at Na channels and 1,4-dihydropyridine, benzodiazepines, and phenylalamines at Ca+2 channels. The most exciting presentations delineated the anti-cancer potential of R-flurbiprofen and the central versus peripheral antinociceptive activity of the enantiomers of aryl propionic acid (APA) class nonsteroidal antiinflammatory drugs (NSAIDs). In addition, the mechanism by which R-APAs contribute to the ulceroginicity of their enantiomers was hypothesized. J. D. McCracken of Lorna Linda (see pp. 540-545) described how agents that decrease cell proliferation in the crypts of Lieberkuhn have a protective effect against colon cancer. Although NSAIDs in general reduce the labeling index of these cells, other gastrointestinal effects (including ulceration) may limit their use in prevention. This effect has been assumed to be associated with inhibition of Cox 1 and/or 2. Although R-flurbiprofen does not significantly inhibit either of these enzymes, it was as effective as S-flurbiprofen in