Reversible binding of tolmetin, zomepirac, and their glucuronide conjugates to human serum albumin and plasma

Exploring the binding interactions of bicalutamide with bovine serum albumin: spectroscopic techniques and molecular modeling studies

The current study employed a variety of spectroscopic methods and molecular modeling to thoroughly look at, under physiological settings, the interaction between bicalutamide (BIC) and bovine serum albumin (BSA). According to our study, the BSA-BIC system's static quenching procedure is supported by the Stern-Volmer quenching constants. The binding constant dropped with temperature, implying that the BSA-BIC complex was weakened. The BSA absorption spectra shifted to a lower wavelength area (from 278 to 272 nm) upon the addition of BIC. The distance (r) between the acceptor and donor in the complex of BIC-BSA and circular dichroism (CD) spectra show the molecular exchanges between BIC and BSA. This study is essential for understanding the therapeutic approach to cancer management through drug distribution and pharmacological effectiveness.

Extrapolation of diclofenac clearance from in vitro microsomal metabolism data: role of acyl glucuronidation and sequential oxidative metabolism of the acyl glucuronide

Diclofenac is eliminated predominantly (approximately 50%) as its 4'-hydroxylated metabolite in humans, whereas the acyl glucuronide (AG) pathway appears more important in rats (approximately 50%) and dogs (>80-90%). However, previous studies of diclofenac oxidative metabolism in human liver microsomes (HLMs) have yielded pronounced underprediction of human in vivo clearance. We determined the relative quantitative importance of 4'-hydroxy and AG pathways of diclofenac metabolism in rat, dog, and human liver microsomes. Microsomal intrinsic clearance values (CL(int) = V(max)/K(m)) were determined and used to extrapolate the in vivo blood clearance of diclofenac in these species. Clearance of diclofenac was accurately predicted from microsomal data only when both the AG and the 4'-hydroxy pathways were considered. However, the fact that the AG pathway in HLMs accounted for ~75% of the estimated hepatic CL(int) of diclofenac is apparently inconsistent with the 4'-hydroxy diclofenac excretion data in humans. Interestingly, upon incubation with HLMs, significant oxidative metabolism of diclofenac AG, directly to 4'-hydroxy diclofenac AG, was observed. The estimated hepatic CL(int) of this pathway suggested that a significant fraction of the intrahepatically formed diclofenac AG may be converted to its 4'-hydroxy derivative in vivo. Further experiments indicated that this novel oxidative reaction was catalyzed by CYP2C8, as opposed to CYP2C9-catalyzed 4'-hydroxylation of diclofenac. These findings may have general implications in the use of total (free + conjugated) oxidative metabolite excretion for determining primary routes of drug clearance and may question the utility of diclofenac as a probe for phenotyping human CYP2C9 activity in vivo via measurement of its pharmacokinetics and total 4'-hydroxy diclofenac urinary excretion.

Human serum albumin binding of novel antiretroviral nucleoside derivatives of AZT

The binding of novel nucleoside derivatives (2-7) to the Human Serum Albumin (HSA) was studied using zidovudine (AZT), as standard compound. The applicability of two different techniques to separate unbound drug from drug-protein complex was analyzed: the gel filtration and ultrafiltration methods. Ultrafiltration was found to be an adequate procedure for the separation of unbounded drug from the drug-protein complex. Incubation temperature ranging from 0 to 37 degrees C did not modify considerably the bound fractions. The same effects were observed as HSA concentration was modified. Binding assays of studied compounds to purified 1% (w/v) HSA at 0 degrees C, indicate that bound fraction of 2-7 ranges from 13 to 47%, exhibiting a higher affinity to HSA than AZT (12%), which would introduce some interesting improvements in their pharmacokinetic properties. In addition, by means of displacement studies using HSA site specific drugs such as diazepam and salicylate, it was determined that AZT binds to site I of the HSA molecule, by a mainly entropy driven process (DeltaS = 10.834 cal/mol degrees K), being these observations extensive to 2-7. Some structural basis to explain enhanced affinity of these novel derivatives was also established.

Induction of cytokine release by the acyl glucuronide of mycophenolic acid: a link to side effects?

OBJECTIVES

We have identified an acyl glucuronide (M-2) of the immunosuppressant mycophenolic acid (MPA). Acyl glucuronides have toxic potential and may contribute to drug toxicity. Whether acyl glucuronides are able to induce release of proinflammatory cytokines is unknown. Gastrointestinal disturbances have been observed during MPA therapy and may involve an inflammatory reaction. This study investigated whether M-2 can induce IL-6 and TNF-alpha release as well as gene expression of these cytokines in leukocytes.

DESIGN AND METHODS

M-2 was produced by incubation of MPA with human liver microsomes. Human mononuclear leukocytes were incubated in the presence of M-2. Concentrations of IL-6 and TNF-alpha were measured by ELISA. Expression of mRNA was determined by quantitative RT-PCR.

RESULTS

Incubation of 3 x 10(6) cells with M-2 resulted in a time and dose dependent release of cytokines, whereas MPA or its phenolic glucuronide MPAG were without effect. Cytokine liberation depended on mRNA induction. Response to M-2 showed much inter individual variability (30-fold for IL-6, 3-fold for TNF-alpha).

CONCLUSIONS

If M-2 promotes release of cytokines in vivo, these may mediate some of the toxic actions of MPA.

Identification of uridine diphosphate glucuronosyltransferases involved in the metabolism and clearance of mycophenolic acid

Mycophenolic acid, the active metabolite of the immunosuppressant and antiproliferative agent, mycophenolate mofetil, is primarily metabolized by glucuronidation to the inactive 7-O-glucuronide. Although the uridine diphosphate (UDP) 7-O-glucuronide is the principal excretion product of this drug, carboxyl-linked glucuronides have also been detected in vitro and in vivo. To identify human UDP glucuronosyltransferases that are active in the glucuronidation of mycophenolic acid, cDNAs encoding individual UDP glucuronosyltransferase forms have been expressed in cell culture, and the capacity of the expressed enzymes to use mycophenolic acid as a substrate has been assessed. Two UDP glucuronosyltransferase forms, UGT1A8 and UGT1A10, were active in the glucuronidation of mycophenolic acid. Both enzymes are predominantly expressed in the gastrointestinal tract and hence, may play a role in the metabolism of mycophenolic acid in the gastrointestinal tract and in the acquisition of resistance to the mito-inhibitory effects of this drug in cultured human colorectal carcinoma cell lines. The identities of the UDP glucuronosyltransferase forms that are mainly responsible for the glucuronidation of mycophenolic acid in the liver and kidney remain unknown; however, UGT1A9 may be important in this respect as the cDNA-expressed enzyme has some capacity to glucuronidate mycophenolic acid. Other UGT1A forms in the liver and kidney (UGT1A1, UGT1A3, UGT1A4, and UGT1A6) were inactive toward mycophenolic acid.

Study of the binding of antibiotics to human serum albumin by charge-transfer chromatography

The interaction of 13 antibiotics with human serum albumin was studied by charge-transfer reversed-phase thin-layer chromatography in neutral, acidic, basic and ionic environments (NaCl and MgCl2) and the relative strength of interaction was calculated. The pH and the presence of mono- and divalent cations markedly influenced the strength of interaction. The capacity of antibiotics to interact with HSA also considerably depended on their chemical structure.

Serum protein binding of nonsteroidal antiinflammatory drugs: a comparative study

The unbound fraction in serum, fu, is a critical parameter in describing and understanding the pharmacokinetics of NSAIDs. We compared fu for 6 different NSAIDs using ultrafiltration of pooled serum at pH 7.4 and 24C. Measurements covered a wide concentration range in order to define binding affinity and number of binding sites. HPLC was used to measure drug concentrations in serum and ultrafiltrate. Direct injection of ultrafiltrate and serum (diluted 250 x) permitted quantitation down to approximately 70 nM for most of the NSAIDs, i.e., approximately 15-20 ng/ml. Assuming binding only to albumin, the data were fitted to a model of two classes of binding sites with dissociation constants K1 and K2. The lowest K1 (highest affinity) was found with flurbiprofen, 0.0658 microM, the highest with ketoprofen, 5.23 microM, an 80-fold difference. At low drug concentrations, fu becomes virtually constant and approaches a lower limit, fumin. The following fumin values were calculated: diclofenac 0.21%; fenoprofen 0.25%, flurbiprofen 0.022%, ketoprofen 0.52%, naproxen 0.039%, and tolmetin 0.37%. Thus the least bound NSAID, ketoprofen, had a value 24-fold that of the most highly bound, flurbiprofen. The NSAIDs also differed widely with regard to the extent of variation in fu within the range of therapeutic concentrations, and hence with regard to their potential as displacers of other drugs.

Stereoselective binding properties of naproxen glucuronide diastereomers to proteins

The stability of naproxen glucuronide (NAP-G) diastereomers was investigated in buffer, 0.3% and 3% human serum albumin (HSA) solutions, and human plasma. R-NAP-G was found to be less stable in phosphate buffer than its S-diastereomer, whereas incubation media containing protein in general increased the degradation rate of NAP-G but also caused a change of the stereoselective stability where the R-NAP-G was more stable than S-NAP-G. Reversible binding of NAP-Gs to HSA (0.3%) was investigated and compared with the corresponding properties of naproxen (NAP) enantiomers. NAP-G diastereomers exhibited a considerable and stereoselective affinity to HSA, although less than that observed for the NAP enantiomers. In vitro irreversible binding of NAP-Gs to HSA, human and rat plasma proteins was also investigated. Irreversible binding was higher for R-NAP-G (50 microM) than for S-NAP-G (50 microM) in all incubation media. This stereoselective difference was observed with HSA containing medium as well as in rat and human plasma. Incubation with unconjugated NAP did not lead to irreversible binding. Preincubation of HSA with acetylsalicylic acid (approximately 11 mM) and glucuronic acid (50 mM) decreased the extent of irreversible binding suggesting involvement of lysine residues for covalent binding. Preincubation with S-NAP also decreased the irreversible binding yield.

Idiosyncratic liver toxicity of nonsteroidal antiinflammatory drugs: molecular mechanisms and pathology

This review explores the clinical hepatic pathology associated with the use of nonsteroidal antiinflammatory drugs (NSAIDs), possible cellular and molecular mechanisms of injury, and future challenges. NSAIDs comprise a group of widely used compounds that have been associated with rare adverse reactions in the liver, including fulminant hepatitis and cholestasis. These reactions are idiosyncratic, mostly independent of the dose administered, and host-dependent. The mechanisms responsible for the initiation and perpetuation of NSAID-induced hepatotoxicity remain poorly understood and have been largely inferred from clinical manifestation. A mounting body of evidence, however, indicates that many acidic NSAIDs are metabolized to reactive acyl glucuronides that can form covalent adducts with plasma proteins and hepatocellular proteins. In hepatocytes cocultured with lymphocytes, these NSAID-altered proteins can become antigenic. Thus, long-lived, drug-altered proteins may act as immunogens and produce cytotoxic T-cell-mediated or antibody-dependent, cell-mediated toxicity in susceptible patients. Alternatively, individual abnormalities in metabolism or disposition of some NSAIDs may lead to the formation or accumulation of toxic metabolites. Additional work with transgenic animal models is needed to permit better understanding of the general and specific risk factors involved in the pathogenesis of the idiosyncratic liver injuries related to NSAIDs and other drugs.