Naproxen conjugation

Many nonsteroidal anti-inflammatory drugs (NSAIDs) are substituted 2-arylpropionic acids. Most NSAIDs also have a chiral carbon next to the carboxylate and are administered as a racemic mixture of the two enantiomers. In general, the (S)-enantiomcr is responsible for most of the antiinflammatory activity of these agents. It was found that the (/ -enantiomer is converted to the (S)-enantiomer but the reverse does not occur (23). As with amino acid conjugation, the pathway involves reaction with ATP to form an AMP ester, which is, in turn, converted to a Co-A ester, and it is the Co-A ester that undergoes chiral inversion (Fig. 7.14). Substrates include ibuprofen, naproxen, and fenoprofen. 

We have developed albumin-conjugates of dexamethasone and naproxen for targeting to SECs and KCs. These conjugates have been extensively tested in vitro and in vivo in rats with bile duct hgation-induced hver fibrosis [93,147-150]. 

The different aspects of drug targeting using LMWPs that have been studied to date are discussed below. As an example, we use the data of two conjugates, naproxen-lysozyme and captopril-lysozyme. 

Targeting of nonsteroidal anti-inflammatory drugs (NSAIDs) such as naproxen could be of interest for the treatment of proteinuria and tubular defects such as Fanconi syndrome and Bartter s syndrome [73,74]. Although a conjugate with an ester spacer is preferred to a conjugate with a direct peptide hnkage [66,67], we continued our research using naproxen di-... 

Figure 5.9. The concentration-time course of (a) naproxen and (b) captopril in the kidney after intravenous injection of the parent drug or the drug-lysozyme (LZM) conjugate. Values are given as means + SEM.

A spacer was used to link captopril via a disulfide bond to the LMWP lysozyme. Conjugation of captopril to lysoz5me resulted in a 6-fold increase in captopril accumulation in the rat kidney (Eigure 5.9b) [77]. This modest enrichment, as compared to that achieved with naproxen-lysozyme, was due to fact that, in contrast to naproxen, free captopril is cleared very efficiently by the kidney itself. Thus, delivery via lysozyme reabsorption only leads to a limited improvement of renal accumulation of captopril. 

No detectable amounts of naproxen or its lysine conjugates were found in the plasma after administration of the conjugate and it can be inferred that excretion into the urine is the crucial process which determines the elimination rate fi. The lack of diffusion into the bloodstream is a favourable property in relation to unwanted extra-renal effects. 

Other drugs are metabolised by Phase II synthetic reactions, catalysed typically by non-microsomal enzymes. Processes include acetylation, sulphation, glycine conjugation and methylation. Phase II reactions may be affected less frequently by ageing. Thus according to some studies, the elimination of isoniazid, rifampicin (rifampin), paracetamol (acetaminophen), valproic acid, salicylate, indomethacin, lorazepam, oxazepam, and temazepam is not altered with age. However, other studies have demonstrated a reduction in metabolism of lorazepam, paracetamol (acetaminophen), ketoprofen, naproxen, morphine, free valproic acid, and salicylate, indicating that the effect of age on conjugation reactions is variable. 

Despite the resolving power of TLC-MS-MS, few applications in drug residue analysis have been reported. One application concerns the HPTLC-MS-MS analysis of a number of nonsteroidal anti-inflammatory drugs, including salicylic acid and its glycine conjugate salicylhippuric acid, diclofenac, indomethacin, naproxen, phenacetin, and ibuprofen (67). Another application describes the detection and identification of some of these compounds or their metabolites in urine by TLC-MS-MS (67). 

Figure 4.7 Typical 2D LC-NMR stop-flow data, showing a TOCSY spectrum obtained on the glucuronide conjugate of Naproxen from a sample of human urine...

Melgert et al. studied the delivery of the corticosteroid dexamethasone to fibrotic livers [240], Dexamethasone has more potent and broader anti-inflammatory effects compared with naproxen. It inhibits the release of inflammatory mediators like TNF-a, IFN-y, and IL-6 and acts as an NFkB inhibitor [241, 242], Dexamethasone coupled to albumin (Dexa-HSA) was specifically taken up by sinusoidal cells in fibrotic rat livers, whereas dexamethasone itself was mainly taken up by hepatocytes. In vivo, Dexa-HSA promoted survival in endotoxin-induced liver inflammation in rats [240], In vitro, anti-inflammatory effects of the conjugate were measured in endotoxin-challenged liver slices. Dexa-HSA inhibited the release of nitric oxide and TNF-a in a dose-dependent manner (Melgert et al. unpublished data). To further enhance the delivery to KC at present dexamethasone is coupled to manHSA, and this conjugate is studied with respect to the pharmacokinetic profile and pharmacotherapeutic effects in fibrotic rats. 

C. Albrecht, R. Reichen, J. Visser, D. K. F. Meijer, and W. Thormann, Differentiation between naproxen, naproxen-protein-conjugates and naproxen-lysine in plasma via micellar electrokinetic capillary chromatography—A new approach in the bioanalysis of drug targeting preparations, Clin. Chem. 43 2083-2090 (1997). 

C. Albrecht, B. N. Melgert, J. Reichen, K. Poelstra, and D. K. F. Meijer, Effect of chronic bile duct obstruction and LPS upon targeting of naproxen to the fiver using naproxen-albumin conjugate, J. Drug Target 6 105-117 (1998). 

Other compounds producing some inhibition of ZDV conjugation were oxazepam, salicylic acid, and acetylsalicyclic acid. More recently, Trapnell et al. examined the inhibition of ZDV at a more relevant concentration of 20 pM in bovine serum albumin (BSA)-activated microsomes by atovaquone, methadone, fluconazole, and valproic acid at therapeutically relevant concentrations (127). Both fluconazole and valproic acid inhibited ZDV glucuronidation by more than 50% at therapeutic concentrations. Clinical interaction studies have been conducted with methadone, fluconazole, naproxen, probenecid, rifampicin, and valproic acid (see Table 10). 

The conjugates of arylpropionic acids enantiomers with D-glucuronic add were separated on a Beckman Ultrasphere ODS column (4.6 mm x 250 mm, 5 / m) column. The mobile phase was a 28 72 (v/v) mixture of acetonitrile and 8 mM tetrabutylammonium hydrogen sulfate buffer (pH 2.5). Five minutes after the diastereomeric conjugates had been resolved, the percentage of acetonitrile was increased to 60% to wash out excess substrate. The mode of detection depended on the samples injected. Fluorescence was used for flunoxaprofen (excitation, 305 nm emission, 355 nm), benoxaprofen (313/ 365 nm) carprofen (285/350 nm), and idoprofen (275/433 nm). UV absorption was used to detect flurbiprofen (255 nm), naproxen (285 nm), ketoprofen (255 nm), pirprofen (265 nm), and cicloprofen (238 nm). 

Disposition in the Body. Readily and almost completely absorbed after oral or rectal administration. About 50% of a dose is excreted in the urine in 24 hours and about 94% in 5 days together with about 1 to 2% in the faeces. Of the material excreted in the urine, approximately 60% is conjugated naproxen, 5% is 6-0-desmethylnaproxen, and 20% is conjugated des-methylnaproxen. Less than 10% of the excreted material is unchanged drug. 

Hydrolysis of conjugated naproxen has been reported during storage of urine samples, and assays carried out immediately after collection suggested that only very small amounts of unchanged drug were excreted (R. A. Upton etal., J. pharm. Sci., 1980, 69,1254-1257). 

---