Protein binding of ceftriaxone in extravascular fluids

Ceftriaxone binding in extravascular fluids and diluted plasma was characterized by equilibrium dialysis techniques and the data was subjected to Scatchard analysis. The extent of ceftriaxone binding in extravascular fluids (synovial, lymph, ascites, and pleural exudate) was less than plasma due primarily to lower albumin concentrations. Ceftriaxone capacity constants were highly correlated (r2 = 0.900, p less than 0.001) with measured albumin concentrations (range of 43 g/L for albumin to 4.7 g/L for one of the pleural exudate samples). The binding affinity constant (M-1 x 10(-4] was comparable for plasma (3.67), synovial fluid (4.14), and lymph (3.40), but was lower for ascites (2.37) and pleural fluid (range of 2.77 to 0.31). Plasma samples diluted with plasma water (range of 100 to 3%) exhibited a common affinity constant (mean value 4.03 x 10(4) M-1) and capacity constants which correlated directly with albumin concentration (r2 = 0.998, p less than 0.001). Analysis of these observations suggests that extravascular binding of ceftriaxone can readily be predicted if extravascular albumin concentration and corresponding disease-state plasma protein binding are known.     

Protein binding of non-steroidal anti-inflammatory drugs in plasma and synovial fluid of arthritic patients

1 The protein binding of seven non-steroidal anti-inflammatory drugs (indomethacin, tolmetin, salicylic acid, ibuprofen, flurbiprofen, naproxen and GP53,633) and warfarin was investigated by equilibrium dialysis in simultaneous samples of synovial fluid and plasma from 12 arthritic patients. 2 The protein binding of all drugs studied except warfarin and flurbiprofen was significantly lower in synovial fluid than in plasma. 3 The decreased protein binding of these drugs is likely to explain the lower total drug concentrations found in synovial fluid in comparison to plasma. 4 The lower albumin concentration plays an important role in determination of reduced drug binding in synovial fluid compared to plasma and the fatty acid concentration in synovial fluid may also influence the protein binding of some of these drugs.     

Distribution of moricizine in human blood: binding to plasma proteins and erythrocytes.

Using equilibrium dialysis and incubation experiments, we determined the binding of moricizine to human plasma, isolated plasma proteins, and erythrocytes. The mean (% +/- SD) plasma protein binding at various moricizine concentrations ranged from 81.2 +/- 2.1 to 89.9 +/- 2.1%. There was no apparent relationship between drug concentration and extent of binding in pooled plasma over the concentration range tested. However, protein concentration-dependent binding was observed with albumin and alpha 1-acid glycoprotein (alpha 1-AGP). The unbound fraction of moricizine fell from 61 to 19% and from 70 to 17% with increasing albumin (5 and 50 g/L, respectively) and alpha 1-AGP (0.2 and 1.2 g/L) concentrations. The binding of moricizine to beta-lipoprotein (5 g/L) was 70.6 +/- 3.1% and to gamma-globulin (12 g/L) was 13.6 +/- 3.3%. Moricizine partitioned into erythrocytes, showing an erythrocyte/plasma drug concentration ratio of 1.325 +/- 0.070 and erythrocyte/buffer ratio of 8.561 +/- 0.620. An estimation could be made that 57% of total drug in whole blood was associated with erythrocytes, 39% bound to plasma proteins, and 4% was free. The results of this study demonstrated that erythrocytes, albumin, and alpha 1-AGP were the major binding components in blood.     

In vitro protein binding behavior of dipyridamole

The in vitro protein binding behavior of dipyridamole in plasma and buffered protein solutions was investigated by equilibrium dialysis. The drug was highly protein bound (approximately 98%) in heparinized human plasma, and the extent of protein binding remained constant for drug concentrations over the range of therapeutic interest of 0.1-10 micrograms/mL. Comparable binding results were obtained with a mixture of 80 mg % of alpha 1-acid glycoprotein and 40 g/L of human serum albumin in pH 7.4 phosphate buffer solution. Pure alpha 1-acid glycoprotein (80-400 mg %) or pure human serum albumin (40 g/L) in phosphate buffer gave significantly (p less than 0.05) lower binding results, indicating that both proteins are responsible for the high binding of dipyridamole in plasma. Addition of alpha 1-acid glycoprotein to heparinized human plasma, to simulate an acute phase increase in the protein, had no effect on the fraction of free drug in plasma. Binding of dipyridamole to heparinized human plasma or human serum albumin in buffer was concentration independent through 40 micrograms/mL. The free fraction of dipyridamole increases with concentrations exceeding 40 micrograms/mL.     

Plasma and synovial fluid kinetics of flurbiprofen in rheumatoid arthritis.

Clinical assessment, plasma and synovial fluid kinetics were studied in 29 rheumatoid patients receiving 100 mg flurbiprofen twice daily. Clinical assessment and pharmacokinetic measurements varied widely within the group of patients. The average values for plasma clearance, volume of distribution and elimination halflife of flurbiprofen were 0.65 +/- 0.24 ml min-1 kg-1, 0.160 +/- 0.093 l kg-1 and 3.1 +/- 1.7 h, respectively. Synovial fluid drug concentrations peaked later and were lower than corresponding plasma concentrations: 5.2 h and 4.4 mg l-1 as against 1.49 h and 12.5 mg l-1, respectively. At 48 h after an oral dose of flurbiprofen, all the drug had been cleared from the synovial fluid. Synovial fluid drug concentrations were not related to synovial fluid albumin concentration or pH. There was a weak relationship between synovial fluid drug concentration and the thermographic measurements of disease activity. The fractions of flurbiprofen not bound to protein in synovial fluid and plasma were not significantly different. A simple model is proposed to account for the plasma and synovial fluid pharmacokinetics.     

Stereoselective disposition of etodolac enantiomers in synovial fluid.

The synovial fluid (SF) uptake of the chiral nonsteroidal anti-inflammatory drug, etodolac, was studied in six arthritic patients, 2 hours (n = 1) or 12 hours (n = 5) after a single 200 mg dose of racemate. Marked stereoselectivity was seen in both SF and plasma; concentrations of pharmacologically inactive R-etodolac were up to 10-fold greater than active S-etodolac. Concentrations of S-etodolac were greater in SF than in plasma (SF:plasma ratio = 1.98 +/- 0.8): No such difference was seen for R-etodolac (SF:plasma = 0.91 +/- 0.3). Considerable concentrations of conjugated enantiomers were present in SF. In vitro equilibrium dialysis studies in drug-spiked samples showed that the unbound fraction of both enantiomers in SF was greater than in plasma; both fluids bound R more extensively than S etodolac. Therapeutically active S-etodolac has greater concentrations in synovial fluid than plasma during the post-distributive phase, which may be of possible clinical relevance.     

Articular diffusion of meloxicam after a single oral dose: relationship to cyclo-oxygenase inhibition in synovial cells

OBJECTIVE: To investigate the distribution of meloxicam in the human knee joint and to compare it with the inhibition of cyclo-oxygenase (COX) activity in synovial cells. DESIGN: Prospective pharmacokinetic study and in vitro laboratory investigation. PATIENTS and PARTICIPANTS: 42 male and female patients aged 26 to 85 years hospitalised for rheumatic disease and requiring a diagnostic and/or therapeutic knee puncture. METHODS: After a single oral dose of meloxicam 15mg, synovial fluid and blood samples were collected once per patient at various intervals after administration. Meloxicam concentrations were determined by a validated high performance liquid chromatography assay, protein binding by equilibrium dialysis, and pharmacokinetic parameters were calculated by noncompartmental analysis from the mean drug concentration-time profiles. The inhibitory effect of meloxicam on COX activity was investigated separately in unstimulated or interleukin-1beta-stimulated human synovial cells from osteoarthritic patients. RESULTS: Meloxicam was found in synovial fluid at the earliest sampling time (1 hour). Peak concentrations were reached approximately 6 hours postdose in both plasma (842 microg/L) and synovial fluid (320 microg/L). A plateau was observed after the distribution phase (6 hours), corresponding to a constant ratio of drug concentration between synovial fluid and plasma of about 0.47. This ratio was higher in patients with acute inflammation (0.58) than in those with no inflammation (0.38). Meloxicam was extensively bound to protein, mainly to serum albumin. The area under the drug concentration-time curve (AUC) in plasma was more than 2.5 times that in synovial fluid. The AUC for free meloxicam was similar in plasma and synovial fluid. The 50% inhibitory concentrations (IC50) for basal and stimulated COX activity in human synovial cells were 33.7 nmol/L (11.8 microg/L) and 2.0 nmol/L (0.70 microg/L), respectively. The free concentration of meloxicam in synovial fluid was higher than the IC50 for stimulated COX activity from 6 to 36 hours postdose. CONCLUSION: On the basis of free synovial concentrations and the IC50 for stimulated COX activity, meloxicam is expected to have a long duration of action. Inhibition of COX activity is expected to be more marked in inflamed synovium compared with non-inflamed synovium.     

Fluid shifts and other factors affecting plasma protein binding of prednisolone by equilibrium dialysis

The effects of drug stability, radioactive tracer purity, buffer composition, protein concentration, and fluid shifts on the nonlinear plasma protein binding of prednisolone were examined by equilibrium dialysis. Prednisolone exhibits a concentration-dependent degradation; however, the limited extent of this does not affect protein binding. Impure tritiated prednisolone used as a tracer produces incorrect, low fractional binding values with the binding parameters generated for transcortin affected more than those for albumin. Isotonic sodium phosphate and Krebs original Ringer phosphate buffers yield similar fractional binding of prednisolone and identical protein binding parameters. Fractional binding of the steroid decreases with total plasma protein concentration, but the association constants remain constant over a twofold dilution of plasma proteins. Further dilution increases these parameters. A time-dependent colloidal osmotic fluid shift during dialysis causes dilution of plasma protein concentrations and diminished drug binding. Theoretical simulations show that the osmotic fluid shifts produce the largest changes in fractional binding for compounds that are bound by low-capacity proteins with low association constants (K less than 10(6) M-1). A mathematical equation was developed to correct bound drug concentrations and fraction bound for protein dilution caused by this effect. The fluid shifts can be prevented by the addition of dextran (mol. wt. 70,000) to the dialysis buffer in a concentration of 55% of the total protein concentration. Multiple factors can diminish the nonlinear prednisolone binding as artifacts during equilibrium dialysis, but the changes are relatively modest.     

Protein binding of oxazepam and its glucuronide conjugates to human albumin

The binding of oxazepam and its glucuronide conjugates to human serum albumin (HSA), as well as the binding interactions of the drug and its metabolites, were examined by equilibrium dialysis and kinetic probe studies. Oxazepam and its S(+) glucuronide are bound to the HSA molecule with affinity constants of 3.5 X 10(5) M-1 and 5.5 X 10(4) M-1, respectively, which were independent of protein concentration over a range of 0.1 to 5.0 g/dl. The R(-) glucuronide bound weakly to albumin, with the binding parameter, N X K, increasing at lower albumin concentrations. Pre-acetylation of fatty acid free-HSA resulted in decreased binding of all three compounds, probably by altering the conformation of the binding sites. Kinetic probe studies with p-nitrophenyl acetate indicate that oxazepam and its S(+) glucuronide shared a common binding site on HSA, but that the R(-) glucuronide bound at another site. Oxazepam binding was unaffected by the presence of its glucuronide conjugates but was inhibited by fatty acids. The percentage of oxazepam bound to plasma proteins in patients with renal impairment (94%) was lower than in normal volunteers (97%). This lower binding can neither be attributed to lower albumin concentrations because of the large binding capacity of the protein and linearity of N X K nor to displacement by elevated concentrations of glucuronide conjugates, but it may be ascribed partly to increased plasma fatty acids.     

Differences in the binding of quinine and quinidine to plasma proteins.

1. Little is known about the comparative plasma protein binding of the antimalarial agents quinine (QN) and its isomer quinidine (QD). We have examined the in vitro binding of QN and QD to albumin, alpha 1-acid glycoprotein, normal human plasma, and maternal and foetal umbilical cord plasma. 2. QN was more avidly bound than QD, and binding of both drugs was substantially higher to alpha 1-acid glycoprotein than to albumin, indicating that alpha 1-acid glycoprotein is the more important binding protein. 3. Protein and drug concentration dependent binding was evident for both QN and QD. The unbound fraction of both drugs fell with increasing albumin (10 to 60 g l-1) and alpha 1-acid glycoprotein (0.5 to 2.0 g l-1) concentration, and there was a marked increase in unbound fraction of QN (6 to 19%) and QD (13 to 36%) in human plasma when drug concentrations were increased over the antimalarial therapeutic range (0.5 to 10 mg l-1). 4. In human volunteer plasma, the unbound fractions of QN and QD were 7.5 +/- 2.2% and 12.3 +/- 2.3% respectively, whilst the unbound fractions for both drugs were significantly higher in maternal plasma (QN = 13.0 +/- 5.4%, QD = 18.3 +/- 2.5%) and significantly higher still in foetal umbilical cord plasma (QN = 25.7 +/- 10%, QD = 35 +/- 5.3%).     

Diclofenac concentrations in synovial fluid and plasma after cutaneous application in inflammatory and degenerative joint disease.

1. Ten patients with bilateral knee joint effusions were treated topically with a gel containing 1 g diclofenac/100 g (80 mg three times daily). They were randomized to receive diclofenac gel to one knee and a placebo gel preparation to the other knee. 2. Diclofenac was assayed in synovial fluid and blood plasma by GC/ECD as the pentafluorobenzyl-ester derivative. 3. Total concentrations of diclofenac in synovial fluid (day 4) were significantly higher in the diclofenac gel treated knee than in the contralateral placebo treated knee (25.5 +/- 3.6 ng ml-1 vs 21.6 +/- 2 ng ml-1; P less than 0.05). These concentrations were lower than total plasma drug concentrations (40.6 +/- 4.7 ng ml-1, n = 10, P less than 0.01). Unbound concentrations of diclofenac in synovial fluid from either the diclofenac gel treated or the placebo treated knee were not significantly different from each other or from plasma free concentrations (115 +/- 16 and 99 +/- 12 vs 108 +/- 19 pg ml-1). 4. Clinical parameters showed improvement of joint mobility and a small reduction of swelling (circumference) in both knees with time. However, the differences between knees were not significant. 5. We conclude that direct transport of diclofenac from the skin into the ipsilateral knee joint after cutaneous application is minimal. Distribution seems to be predominantly via the blood. Whether the observed improvements of clinical parameters were due to drug effects or to the spontaneous course of the underlying disease cannot be distinguished.     

Characterization of salicylate binding to synovial fluid and plasma protein in patients with rheumatoid arthritis

Summary Protein binding of salicylate in synovial fluid and plasma from patients with rheumatoid arthritis was studied by equilibrium dialysis. Protein binding in the synovial fluid was considerably lower at all salicylate concentrations studied (0.07 – 2.2 mM). Scatchard plots of the data were analyzed assuming binding to two classes of binding sites, each plasma sample being diluted to an albumin concentration equal to that in synovial fluid from the same patient. Binding to the primary binding sites was considerably decreased in synovial fluid in comparison with plasma. The affinity of the secondary binding sites was slightly lower. Thus, at a low therapeutic drug concentration, the decreased binding of salicylate to synovial fluid protein in patients with rheumatoid arthritis could mainly be accounted for by decreasing affinity of binding to the primary binding sites.     

吡喹酮对映异构体与血浆蛋白结合的立体选择性

本实验用平衡透析法研究吡喹酮(POT)对映异构体与血浆蛋白结合的立体选择性。当透析液药物初始浓度在1～32μmol/L范围内时,(±)-POT及其对映异构体与血浆蛋白的结合属非饱和类型。无论牛血清白蛋白(BSA)然度为1.47或5.88×10^-4mol/L,(+)-PQT结合能力(nK)均大于(-)-POT,表现较明显的立体选择性。(-)-和(+)-PQT与兔血浆蛋白结合率分别为73.7±4.4和58.3±10.1%(n=8,p<0.05),其立体选择性与BSA结合的选择方向相反。(-)-和(+)-POT与人血浆蛋白结合率分别为81.9±4.2和83.2±6.9%(n=10,P>0.05),无明显立体选择性。POT对映异构体和血浆蛋白结合立体选择性表现种属差异。     

Effects of tramadol on synovial fluid concentrations of substance P and interleukin-6 in patients with knee osteoarthritis: comparison with paracetamol

Both the analgesic drugs tramadol and paracetamol are widely used for the symptomatic therapy of osteoarthritis (OA). The aim of this double-blind, randomised study in patients with knee OA was to compare their effects on synovial fluid concentrations of interleukin (IL)-6 and substance P (SP). Moreover, we evaluated plasma and synovial fluid concentrations of tramadol and its active metabolite (O-desmethyl-tramadol, M1) after oral treatment with this drug. Twenty patients were enrolled. A group of 10 patients received tramadol (50 mg three times a day), and another group of 10 patients were treated with paracetamol (500 mg three times a day) for 7 days. Both drugs significantly reduced the intensity of joint pain. The synovial fluid concentrations of SP were significantly reduced only by the treatment with tramadol. In this group of patients, IL-6 synovial fluid concentrations were slightly, but not significantly, decreased. Paracetamol did not significantly change the synovial fluid concentrations of SP and IL-6. After oral administration, a considerable amount of tramadol was measurable in synovial fluid. Both in plasma and synovial fluid the concentrations of M1 were markedly lower than those of tramadol, with a T/M1 ratio of 14.7+/-4.6 and 9.3+/-3.9, respectively. These data demonstrate that the activity of tramadol may involve the modulation of inflammatory mediators. Moreover, they indicate that after oral treatment with tramadol, both the parent drug and its active metabolite can penetrate into synovial fluid.     

Factors affecting in vitro protein binding of etoposide in humans.

Clinical studies have demonstrated that the plasma protein binding of etoposide, a widely used anticancer drug, is extensive (approximately 94%), highly variable among patients (10-fold range), and significantly related to serum albumin and total bilirubin concentration. The present study was designed to more thoroughly evaluate factors likely to affect etoposide protein binding under controlled in vitro conditions where single variables could be changed. Protein binding was determined using an equilibrium dialysis method with tritiated etoposide. The binding of etoposide was similar in serum or plasma, and heparin had no effect on binding. Etoposide binding decreased with increased pH, but no clinically significant difference was noted within the range of physiologic pH. Etoposide binding evaluated in single-source donor plasma was concentration-dependent over a concentration range of 1 to 250 micrograms/mL. Etoposide binding parameters determined in normal human plasma were characterized by a single class of binding sites of moderate affinity (K = 2.88 +/- 0.47 x 10(4)) and high capacity (nP = 5.07 +/- 0.5 x 10(-4); where n is the number of binding sites). The etoposide binding ratio was significantly correlated with albumin concentration (r2 = 99%, p less than 0.05). The characteristics of etoposide binding in a 4.0-g/dL solution of human serum albumin (K = 3.56 +/- 1.22 x 10(4) and nP = 5.58 +/- 0.16 x 10(-4)) suggest that the single class of binding sites is on albumin. Bilirubin caused a significant decrease in K, consistent with competitive binding, but only at higher bilirubin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharmacokinetics of lumiracoxib in plasma and synovial fluid

BACKGROUND: Lumiracoxib is a new cyclo-oxygenase-2 (COX-2) selective inhibitor in development for the treatment of rheumatoid arthritis, osteoarthritis and acute pain. OBJECTIVE: To investigate the pharmacokinetics of lumiracoxib in plasma and knee joint synovial fluid from patients with rheumatoid arthritis. DESIGN: Open-label multiple-dose study evaluating the steady-state pharmacokinetics of lumiracoxib in plasma and synovial fluid after 7 days of treatment with lumiracoxib 400 mg once daily. PATIENT POPULATION: Males and females aged 18-75 years with rheumatoid arthritis, having moderate to significant synovial fluid effusion of the knee. OUTCOME MEASURES: Following a 7-day washout period for previous nonsteroidal anti-inflammatory drugs, 22 patients (17 female, 5 male) received lumiracoxib 400 mg once daily for seven consecutive days. On day 7, following an overnight fast, a final dose of lumiracoxib was administered and serial blood and synovial fluid samples were collected for up to 28 hours. Lumiracoxib and its metabolites (4'-hydroxy-lumiracoxib and 5-carboxy-4'-hydroxy-lumiracoxib) were measured by validated high performance liquid chromatography-mass spectrometry methods. The steady-state pharmacokinetics of lumiracoxib were evaluated in plasma and synovial fluid by both a population pharmacokinetic model and noncompartmental analysis.RESULTS: Lumiracoxib was rapidly absorbed (peak plasma concentration at 2 hours) and the terminal elimination half-life in plasma was short (6 hours). Lumiracoxib concentrations were initially higher in plasma than in synovial fluid; however, from 5 hours after administration until the end of the 28-hour assessment period, concentrations of lumiracoxib were higher in synovial fluid than in plasma. Peak drug concentration in synovial fluid occurred 3-4 hours later than the peak plasma concentration. The mean steady-state trough concentration of lumiracoxib in synovial fluid (454 microg/L) was approximately three times higher than the mean value in plasma (155 microg/L), and the area under the concentration-time curve from 12 to 24 hours after administration was 2.6-fold higher for synovial fluid than for plasma. Median lumiracoxib protein binding was similar in plasma and synovial fluid (range 97.9-98.3%). Concentrations of 4'-hydroxy-lumiracoxib, the active COX-2 selective metabolite, remained low in comparison with parent drug in both plasma and synovial fluid. The concentration of lumiracoxib in synovial fluid at 24 hours after administration would be expected to result in substantial inhibition of prostaglandin E(2) formation. CONCLUSION: The kinetics of distribution of lumiracoxib in synovial fluid are likely to extend the therapeutic action of the drug beyond that expected from plasma pharmacokinetics. These data support the use of lumiracoxib in a once-daily regimen for the treatment of rheumatoid arthritis.     

Quinidine and dihydroquinidine interactions in human plasma.

The protein-binding characteristics of dihydroquinidine, a known impurity in drug grade quinidine, in human plasma and the effects of dihydroquinidine on quinidine interactions with these plasma constituents were studied by equilibrium dialysis. In the plasma concentration range of 1.75-23.0 mg/liter, dihydroquinidine binding was similar to the binding observed with quinidine. The data suggested the presence of a single class of binding sites for both compounds in the plasma drug concentration range and samples studied. The mean values for the association constant, K, and the total concentration of binding sites, nPt, for dihydroquinidine were 4.75 +/- 0.67 X 10(4) M-1 and 5.78 +/- 0.17 x 10(-5) M, respectively. The corresponding values for quinidine were 4.78 +/- 1.00 x 10(4) M-1 and 5.65 +/- 0.48 x 10(-5) M. In the presence of 5 and 10% (of total alkaloid content) dihydroquinidine, the plasma concentration of unbound quinidine did not change significantly. At a 20% level of dihydroquinidine, however, an increase in unbound quinidine was observed (p less than 0.05). The elevations in free quinidine concentrations were directly related to the level of dihydroquinidine present. The results of this study indicate that the interactions between dihydroquinidine and quinidine for binding sites on human plasma proteins are competitive.     

The stereoselective disposition of the enantiomers of ibuprofen in blood, blister and synovial fluid.

1. A sensitive, stereospecific assay using gas chromatography-mass spectrometry (GC/MS) was established to measure the concentrations of the enantiomers of ibuprofen in small volumes (50 microliters) of blister fluid. 2. The concentrations of the enantiomers in blister fluid, assessed in eight patients, were similar to those in synovial fluid, both fluids behaving as peripheral compartments with respect to plasma. 3. The mean rate constants of transfer of R-ibuprofen into (0.14 +/- 0.06 h-1) and out of (0.20 +/- 0.04 h-1) blister fluid were not significantly different from those for synovial fluid (0.19 +/- 0.12 h-1, 0.34 +/- 0.11 h-1, respectively). Similarly, the mean rate constants of transfer of S-ibuprofen into (0.22 +/- 0.07 h-1) and out of (0.27 +/- 0.08 h-1) blister fluid were not significantly different from those for synovial fluid (0.29 +/- 0.10, 0.36 +/- 0.11 h-1). However, the correlations were poor between the transfer constants for each of the enantiomers between plasma, and both blister and synovial fluid (P > 0.2). 4. The complex rate constant of transfer of S-ibuprofen into blister fluid (0.22 +/- 0.07 h-1) was greater than that of R-ibuprofen (0.14 +/- 0.07 h-1), which may be explained by the lesser protein binding of the S-enantiomer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Determinants of the plasma protein binding of theophylline in health.

1 The plasma protein binding of theophylline was determined after addition of [14C]-theophylline (15 micrograms/ml) to plasma from 24 healthy drug-free volunteers and equilibrium dialysis for 2 h at 37 degrees C. 2 The percentage of drug unbound was 60.0% +/- 2.2% (s.d.) with very little variation between individuals. The binding ratio of theophylline was not significantly related to the plasma albumin or alpha 1-acid glycoprotein (AAG) concentrations but was significantly, although weakly, negatively related to the logarithm of the non-esterified fatty acid concentration (NEFA) (r = 0.443, P less than 0.05). 3 Intravenous administration of heparin (1000 units) caused a significant rise in plasma NEFA concentration and in the percentage of drug unbound in plasma after equilibrium dialysis. 4 In human serum albumin solutions, the binding ratio of theophylline was significantly related to the albumin concentration and at the albumin concentration seen in the 24 normal subjects, the percentage of drug unbound was almost identical. Addition of AAG in physiological concentrations did not enhance theophylline binding but oleic acid, and to a lesser extent palmitic acid, reduced binding significantly. 5 The percentage of theophylline unbound in plasma varied markedly with pH so that at pH7 the percentage unbound was 52% greater than at pH 8. There was no evidence of concentration dependence of binding up to 140 micrograms/ml theophylline. 6 Theophylline appears to bind almost exclusively to albumin and its plasma protein binding varies little in healthy subjects, showing no concentration-dependence over the therapeutic range of concentrations. The binding is affected by pH and by NEFA concentration, however, and these factors may be of greater importance in disease states. Caution should be employed in the use of heparin in studies of plasma protein binding of theophylline.     

Recent findings on the pharmacokinetics of non-steroidal anti-inflammatory drugs in synovial fluid

Synovial fluid concentration is considered to be an important determinant of clinical response to non-steroidal anti-inflammatory drugs (NSAIDs). Trans-synovial transport of these drugs is a process of limited diffusion, governed partly by the pharmacological characteristics of NSAIDs and partly by the properties of the joint and joint space themselves. The studies which report simultaneous pharmacokinetics of NSAIDs in both plasma and synovial fluid compartments are of 2 types: (1) some compare the concurrent concentrations of drugs in plasma and joint fluid after a single administration. These provide pharmacokinetic information: (2) others, which conform more closely to the therapeutic conditions, look at synovial fluid and plasma concentrations after repeated administration of the drug. Recent findings on the pharmacokinetics of NSAIDs in synovial fluid are reviewed. These studies reveal 2 types of NSAIDs, according to their pharmacokinetic behaviour. First, there are NSAIDs with a short or intermediate plasma elimination half-life. These drugs equilibrate rapidly relative to their elimination; their peak synovial fluid concentrations occur later and are lower than those in plasma. Several hours after administration there is crossover of the concentration curves, and beyond this point, concentrations in synovial fluid may exceed those in plasma. During prolonged treatment, the synovial fluid concentrations of these NSAIDs fluctuate to a much lesser extent than plasma concentrations. Secondly, there are NSAIDs with a long plasma elimination half-life; their peak concentration in synovial fluid is also lower and later than that in plasma. At steady-state their concentrations (total and free) in synovial fluid are about half those in plasma. Numerous variables must be taken into account in attempts to correlate synovial fluid NSAIDs concentrations with clinical response, including protein binding and determination of both active metabolites and (eventually) the enantiomers.