Efflux of zidovudine and 2',3'-dideoxyinosine out of the cerebrospinal fluid when administered alone and in combination to Macaca nemestrina.

To determine if there is active efflux of zidovudine (ZDV) and 2',3'-dideoxyinosine (ddI) out of the cerebrospinal fluid (CSF), and if this efflux is saturable, we investigated the steady-state CSF/plasma concentration ratio of the two drugs when administered alone or in combination. Constant-rate infusions of ZDV, ddI or both were administered to seven macaques (Macaca nemestrina) through a chronic venous catheter for a minimum of 28 hr. Antipyrine, a marker of passive diffusion, was coinfused in all experiments. Blood (5 mL) and CSF samples (0.5-1 mL) were collected by venous and lumbar/thoracic punctures, respectively, at 24 and 28 hr after beginning the infusion. When ZDV and ddI were administered alone, the steady-state CSF/plasma concentration ratios were significantly different from unity (ZDV, 0.20 +/- 0.08; ddI, 0.09 +/- 0.04) and were independent of the plasma concentration (P > 0.05). In contrast, the CSF/plasma concentration ratio of antipyrine (0.82 +/- 0.19) was close but significantly smaller than unity (P > 0.05). The CSF/plasma concentration ratios after simultaneous administration of ZDV and ddI were not significantly different (P > 0.05) from those obtained after administration of the drugs alone. These results suggest that ZDV and ddI are actively transported out of the CSF; however, within the concentration range studied, this efflux is neither saturable nor mutually competitive. Concomitant administration of ZDV and ddI did not produce a systemic interaction in the animals, indicating that the pharmacokinetics of either drug is unaffected by the presence of the other.

Disposition of drugs in cystic fibrosis. IV. Mechanisms for enhanced renal clearance of ticarcillin.

To investigate the hypothesis that renal secretion of penicillins is enhanced in cystic fibrosis the maximal tubular secretion rate (Tmax) of ticarcillin and the serum concentration of ticarcillin at half-maximal secretion rate (TC50) were determined in patients with cystic fibrosis (n = 6) and control subjects (n = 6). Each subject received three consecutive constant-rate intravenous infusions of ticarcillin (4, 13, and 70 mg/kg/hr; 2 1/2 hours each) simultaneously with a constant-rate (30 mg/kg/hr) infusion of insulin. Urine samples were collected at 1/2-hour intervals and serum samples at the midpoint of the urine collections. Ticarcillin and inulin concentrations in serum and urine were determined by high-performance liquid chromatographic and a spectrophotometric method, respectively. Ticarcillin serum protein binding was determined by ultrafiltration. Steady-state ticarcillin serum concentrations were achieved at all three infusion rates. The TC50 was significantly lower (p < 0.05) in patients with cystic fibrosis (33.7 +/- 12.2 micrograms/ml) compared with that in control subjects (77.6 +/- 38.4 micrograms/ml). In contrast, the Tmax was similar (cystic fibrosis, 0.25 +/- 0.12 mg/min/kg; control, 0.22 +/- 0.14 mg/min/kg; p > 0.05). These data indicate that renal clearance of penicillins is enhanced in cystic fibrosis because of greater affinity of the renal secretory system for these drugs.

Disposition of drugs in cystic fibrosis. V. In vivo CYP2C9 activity as probed by (S)-warfarin is not enhanced in cystic fibrosis.

Enhanced metabolism of theophylline in subjects with cystic fibrosis suggests that the activity of certain cytochrome P450 isoforms is affected in subjects with this genetic disease. To determine whether this effect on the P450 enzymes is selective, the in vivo activity of the cytochrome P450 isoform CYP2C9 was determined in adult subjects with cystic fibrosis (n = 6) and in control subjects (n = 8). Subjects were administered (S)-warfarin as a single intravenous bolus dose (0.375 mg/kg), and urine and plasma samples were collected for 96 hours. Plasma (S)-warfarin concentrations were determined by HPLC; urinary concentrations of (S)-warfarin and its metabolites were determined by gas chromatography-mass spectrometry. The total plasma clearance of (S)-warfarin (subjects with cystic fibrosis, 3.6 +/- 0.48 ml/hr/kg; control subjects, 3.82 +/- 0.73 ml/hr/kg), elimination half-life (subjects with cystic fibrosis, 29.5 +/- 4.2 hours; control subjects, 25.9 +/- 5.4 hours); and steady-state volume of distribution (subjects with cystic fibrosis, 153 +/- 18 ml/kg; control subjects, 138 +/- 22 ml/kg) were similar in the two groups (p > 0.05). The metabolic clearance of (S)-warfarin to its major metabolites mediated by CYP2C9, 6-hydroxywarfarin and 7-hydroxywarfarin, was not significantly (p > 0.05) different between the two groups (6-hydroxywarfarin: subjects with cystic fibrosis, 0.33 +/- 0.1 ml/hr/kg; control subjects, 0.41 +/- 0.1 ml/hr/kg; 7-hydroxywarfarin: subjects with cystic fibrosis, 1.34 +/- 0.49 ml/hr/kg; control subjects, 1.8 +/- 0.45 ml/hr/kg). On the basis of these data, we conclude that the in vivo cytochrome P450 activity is selectively affected in persons with cystic fibrosis.

In vitro hydrolysis of steroid acid ester derivatives of prednisolone in plasma of different species.

The in vitro hydrolysis of two new classes of steroid acid esters synthesized from prednisolone as local anti-inflammatory steroids was investigated in rat, rabbit, and human plasma. One class was synthesized by incorporating methoxycarbonyl groups at the 16 position of prednisolone to produce 16 alpha-methoxycarbonyl prednisolone (P16CM) and its 17-deoxy analogue (DP16CM). The other class was synthesized by modifying the ketol side chain of prednisolone to produce methyl 20 alpha- and methyl beta-dihydroprednisolonate (P4 alpha and P4 beta). The P16CM and P4 beta were rapidly and completely hydrolyzed within 1 h of incubation in rat and rabbit plasma and within 4 h in human plasma. There was a marked species difference in the hydrolysis of DP16CM which occurred in the following order: rat greater than human greater than rabbit. The in vitro hydrolysis of P4 alpha was much slower than that of P4 beta; the process continued over 24 h in rat plasma. As expected, no change in the initial concentration of prednisolone was found over 120 h of incubation in rat plasma. This marked species difference in the hydrolysis of these steroid acid esters is probably related to the differences in the amounts, types, and activities of the hydrolyzing enzymes (e.g., esterases) in the plasma of the three species. From this study it can be concluded that the existence of an hydroxyl group at C-17 and the orientation of hydroxyl groups at C-20 play an important role in the systemic hydrolysis rate of the carboxy ester group on the steroid nucleus.

Pharmacokinetics of prednisolone and its local anti-inflammatory steroid-21-oate ester derivatives. In vitro and in vivo comparative study.

A new class of local anti-inflammatory steroid-21-oate esters synthesized by modifying the ketol side chain of prednisolone was found to exhibit minimal systemic side effects such as pituitary-adrenal suppression. It has been hypothesized that the absence of the systemic toxicities of these steroids is due to the rapid hydrolysis of the carboxylate ester group to inactive and readily excretable acid metabolites. The pharmacokinetics of prednisolone and its two ester derivatives, methyl 20 alpha- and 20 beta-dihydroprednisolonate (P4 alpha and P4 beta), were studied in rats following im administration of doses ranging from 0.5 to 10 mg/kg. The absorption of each of the three compounds was rapid, as the peak concentration was attained within 20 min after injection. The elimination half-lives of P4 alpha, P4 beta, and prednisolone were independent of dose and were 1.12, 0.28, and 0.53 hr at 10 mg/kg doses, respectively. The AUCs of P4 alpha, P4 beta, and prednisolone following a 10 mg/kg dose were 7678, 242, and 3037 ng/ml.hr, respectively. The pharmacokinetics of the P4 alpha were linear as the AUC increased proportionally with dose. An in vitro study showed that P4 beta was approximately 100% hydrolyzed within 1 hr in plasma at 37 degrees C, whereas P4 alpha was about 30%. However, a negligible disappearance of prednisolone occurred in vitro over a period of more than 2 days. These results suggest that the minimal systemic side effects of these new steroids may be ascribed to the differences in their pharmacokinetic profiles and metabolism from those of prednisolone.

Urinary excretion of prednisolone following intravenous administration in humans.

The urinary excretion of prednisolone was studied in eight normal human volunteers (two women and six men) following intravenous (16, 32, 48 and 64 mg) doses. Urine prednisolone concentrations were determined by a high performance thin layer chromatographic method (HPTLC). The overall mean prednisolone elimination half life in urine following all the intravenous doses as determined by the rate and sigma minus plots was 1.13 +/- 0.25 hour. This was independent of dose and shorter than that found in plasma (4.10 +/- 1.00 s.d. hour). The overall mean percentage of dose excreted unchanged in urine was 16.7 +/- 5.8% following all intravenous and oral doses respectively. About 80% of this amount was excreted within the first 4 hours of the intravenous administration. Renal clearance of prednisolone decreased with time by the first order kinetic (r = 0.790) and its overall value following all IV doses was 0.0183 +/- 0.0103 (s.d.) l/h/kg. The metabolic clearance remained constant with increasing doses from 16 to 64 mg (0.0883 +/- 0.0306 s.d. l/h/kg). From this study it was concluded that a definitive account of the renal elimination of prednisolone and its possible metabolites warrant further investigation. The fraction of the dose excreted unchanged was relatively small and variable suggesting that prednisolone elimination occurs mainly by metabolism.

Two chromatographic methods for the determination of corticosteroids in human biological fluids: pharmacokinetic applications.

Two techniques, high-performance liquid chromatography (HPLC) and quantitative high-performance thin-layer chromatography coupled with densitometry (HPTLC), were developed for the determination of prednisolone (PL), methylprednisolone (MP), and methylprednisolone sodium succinate (MPSS) in human plasma, saliva, and urine. The HPLC and HPTLC methods shared a single and simple step of an organic extraction procedure and separation of steroids using a normal-phase column or HPTLC plate. The methods allow simultaneous measurement of endogenous cortisol in plasma following the administration of PL and MP. The calibration curves of steroids in all biological fluids were linear over a wide range of concentrations of PL and MP in all biological fluids (0.025-4 micrograms/mL). The limit of detection of both assays for PL and MP was 10 ng/mL in plasma and saliva and 25 ng/mL in urine, and of MPSS was 50 ng/mL in plasma. Both methods were reproducible with an inter- and intra-assay coefficient of variation of less than 10% for all steroids over a wide range of concentrations in all biological fluids. No interference from endogenous steroids was found. The presented methods are simple, rapid, specific, sensitive, reproducible, and economical for the pharmacokinetic study of these steroids. The application of these methods for the pharmacokinetic study of both MP and PL in vivo and the in vitro hydrolysis of MPSS is discussed.

Methylprednisolone pharmacokinetics after intravenous and oral administration.

1. The pharmacokinetics of methylprednisolone (MP) were studied in five normal subjects following intravenous doses of 20, 40 and 80 mg methylprednisolone sodium succinate (MPSS) and an oral dose of 20 mg methylprednisolone as 4 x 5 mg tablets. Plasma concentrations of MP and MPSS were measured by both high performance thin layer (h.p.t.l.c.) and high pressure liquid chromatography (h.p.l.c.). 2. The mean values (+/- s.d.) of half-life, mean residence time (MRT), systemic clearance (CL) and volume of distribution at steady state (Vss) of MP following intravenous administration were 1.93 +/- 0.35 h, 3.50 +/- 1.01 h, 0.45 +/- 0.12 lh-1 kg-1 and 1.5 +/- 0.63 1 kg-1, respectively. There was no evidence of dose-related changes in these values. The plasma MP concentration-time curves were superimposable when normalized for dose. 3. The bioavailability of methylprednisolone from the 20 mg tablet was 0.82 +/- 0.11 (s.d.). 4. In vivo hydrolysis of MPSS was rapid with a half-life of 4.14 +/- 1.62 (s.d.) min, and was independent of dose. In contrast, in vitro hydrolysis in plasma, whole blood and red blood cells was slow; the process continuing for more than 7 days. Sodium fluoride did not prevent the hydrolysis of MPSS.

High-performance liquid chromatographic determination of prednisolone and its steroid 21-oate ester derivatives in rat plasma: pharmacokinetic applications.

A high-performance liquid chromatographic (HPLC) technique was developed for the determination of prednisolone and its local anti-inflammatory steroid 21-oate ester derivatives in rat plasma. These new steroid esters (methyl 20 alpha- and 20 beta-dihydroprednisolonate; P4 alpha and P4 beta), developed for local use, were found to exhibit minimal systemic side effects as compared with prednisolone. The described method involves a simple organic extraction procedure and separation of steroids using a C18 reversed-phase column for pharmacokinetic study. The method allows simultaneous measurement of endogenous corticosterone following administration of P4 alpha, P4 beta, and prednisolone. The calibration curves of the steroids were linear over a wide range of concentrations (0.05 to 10 micrograms/mL). The limit of detection of the assay for all tested steroids is 10-20 ng/mL. The method is reproducible, with a coefficient of variation of less than 10% for all steroids over a wide range of concentrations. No interference from endogenous steroids nor exogenous steroids was found. The presented method is simple, rapid, specific, sensitive, and reproducible.

Effect of food on the absorption and pharmacokinetics of prednisolone from enteric-coated tablets.

Prednisolone absorption and bioavailability of 10 mg enteric-coated (EC) and plain (uncoated) tablets were investigated after fasting and heavy meals (EC only) consumed to satiety in normal healthy volunteers. The same volunteers had also received 16 mg of prednisolone intravenously. In fasted subjects, the absolute bioavailability fraction, as normalised for intravenous doses, of prednisolone from plain tablets was 1.055 and from EC tablets was 0.996. The peak concentrations after plain and EC tablets were 309 and 249 ng/ml attained at 0.98 and 5.14 h, respectively. The means plasma elimination half-lives following the plain, EC tablets and intravenous administration in fasting conditions were 3.73, 3.89 and 3.78 h, respectively. Food interfered with both the absorption and the pharmacokinetics of prednisolone after EC tablets resulting in variability in its plasma levels. In some cases absorption of prednisolone was delayed for 12 h and remained at a measurable level for 24 h. In other cases, a normal absorption pattern was observed. This inter- and intrasubject variability of the effect of food appears to be related to its quantity, constituents and also the subjects physiological characteristics. It is concluded that enteric-coated prednisolone tablets should be administered at least 2 h between meals. However, for more predictable corticosteroid absorption (perhaps thus avoiding the therapeutic failure), plain prednisolone tablets are preferable.

Angiotensin-converting enzyme. Investigation of diurnal variation, the effect of a large dose of prednisolone, and prednisolone pharmacokinetics in patients with sarcoidosis.

Serial assay of serum angiotensin-converting enzyme concentrations (SACE) is advocated for monitoring disease progress in sarcoidosis. Because little is known of nondisease factors affecting SACE, 10 patients with histologically proved sarcoidosis were assessed for diurnal fluctuation in SACE and as to whether a large dose of corticosteroid had an immediate effect on SACE independent of disease. The pharmacokinetics of prednisolone in 8 of these patients was also evaluated. On Day 1, serum samples were obtained for 24 h after placebo, and the next day at the same times after 75 mg of orally administered prednisolone. There was no obvious diurnal pattern on either day, and there was no significant difference in SACE after prednisolone. The mean maximal difference obtained within or between days was 8.8%. Prednisolone pharmacokinetics were comparable to normal volunteers. SACE concentrations can be confidently determined at any time of day, and changes of greater than 9% are probably significant. The use of prednisolone in patients with sarcoidosis can be safely based upon pharmacokinetic data obtained from normal volunteers.

Prednisolone elimination in human saliva.

The elimination of prednisolone in saliva was studied following administration of intravenous doses of 16, 32, 48 and 64 mg to 7 healthy human subjects. While there was an approximate relationship between salivary and plasma concentrations of prednisolone a wide inter- and intraindividual variation in the ratio of the concentration in these two body fluids was observed. Salivary estimations of prednisolone concentration cannot replace plasma concentrations in biopharmaceutical studies.

Rifampicin reduces effectiveness and bioavailability of prednisolone.

Rifampicin is an inducer of hepatic drug metabolising enzymes. This results in interactions with several drugs including oral anticoagulants, hypoglycaemics, and contraceptives. Concurrent treatment with prednisolone and rifampicin is given when tuberculosis coexists with a disease that is sensitive to steroids, when the diagnosis is uncertain, or occasionally in the treatment of severe tuberculosis. Two patients with respiratory disease were treated with both drugs: their condition improved considerably after rifampicin was withdrawn. Seven patients were then studied to assess the effect of rifampicin on the pharmacokinetics of prednisolone. Overall, rifampicin increased the plasma clearance of prednisolone by 45% and reduced the amount of drug available to the tissues (area under the plasma concentration time curve) by 66%. The effectiveness of prednisolone may be considerably reduced when rifampicin and prednisolone are used in combination.

Comparison of radioimmunoassay and thin layer chromatographic assay methods for estimation of plasma prednisolone concentrations.

Radioimmunoassay and thin layer chromatographic methods of assay for prednisolone in plasma have been compared. These methods are comparable in terms of ease, speed of execution, and cost. They yielded similar estimates of prednisolone concentration without important bias over the concentration range generally encountered in clinical practice and may be considered comparable for pharmacokinetic studies.

Pharmacokinetics of intravenous and oral prednisolone.

1 Doses of 16, 32, 48 and 64 mg prednisolone were administered intravenously to normal volunteers who also received 100 prednisolone orally. Plasma prednisolone concentrations were estimated by quantitative thin layer chromatography. 2 The bioavailability fraction was 1.063 +/- 0.154 (s.d.) indicating complete availability of prednisolone following oral administration. 3 The mean T 1/2 over all doses were 4.11 +/- 0.97 (s.d.) h and there was no evidence of a dose-related change in its value. 4 The mean systemic clearance over all doses was 0.104 +/- 0.034 (s.d) 1 h-1 kg-1. There was no evidence of a dose-related change in clearance or in the apparent volume of distribution (overall mean 0.588 +/- 0.152 1 kg-1). 5 The area under the plasma concentration-time curve was linearly related to dose. 6 Plasma concentration-time curves normalised for dose were superimposable. 7 It was concluded that over the dose range investigated, non-linear pharmacokinetic behaviour had not been demonstrated in this group of normal volunteers.