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Methylation of histone H3 lysine-4 (H3K4) is an important, regulatory, epigenetic post-translational modification associated with actively transcribed genes. In humans, the principal mediators of this modification are part of the MLL/SET1 family of methyltransferases, which comprises six members, MLLs1-4 and SET1A/SET1B. Aberrations in the structure, expression, and regulation of these enzymes are implicated in various disease states, making them important potential targets for drug discovery, particularly for oncology indications. The MLL/SET1 family members are most enzymatically active when part of a "core complex," the catalytic SET-domain-containing subunits bound to a subcomplex consisting of the proteins WDR5, RbBP5, Ash2L and a homodimer of DPY-30 (WRAD2). The necessity of MLL/SET1 members to bind WRAD2 for full activity is the basis of a particular drug development strategy, which seeks to disrupt the interaction between the MLL/SET1 subunits and WDR5. This strategy is not without its theoretical and practical drawbacks, some of which relate to the ease with which complexes of Escherichia coli-expressed MLL/SET1 and WRAD2 fall apart. As an alternative strategy, we explore ways to stabilize the complex, focusing on the use of an excess of WRAD2 to drive the binding equilibria toward complex formation while maintaining low concentrations of the catalytic subunits. The purpose of this approach is to seek inhibitors that bind the SET domain, an approach proven successful with the related, but inherently more stable, enhancer of zeste homolog 2 (EZH2) complex.

Epigenetic modifications play a crucial role in human diseases. Unlike genetic mutations, however, they do not change the underlying DNA sequences. Epigenetic phenomena have gained increased attention in the field of cancer research, with many studies indicating that they are significantly involved in tumor establishment and progression. Histone methyltransferases (HMTs) are a large group of enzymes that specifically methylate protein lysine and arginine residues, especially in histones, using S-adenosyl-L-methionine (SAM) as the methyl donor. However, in general, HMTs have no widely accepted high-throughput screening (HTS) assay format, and reference inhibitors are not available for many of the enzymes. In this study, we describe the application of a miniaturized, radioisotope-based reaction system: the HotSpot(SM) platform for methyltransferases. Since this platform employs tritiated SAM as a cofactor, it can be applied to the assay of any HMT. The key advantage of this format is that any substrate can be used, including peptides, proteins, or even nucleosomes, without the need for labeling or any other modifications. Using this platform, we have determined substrate specificities, characterized enzyme kinetics, performed compound profiling for both lysine and arginine methyltransferases, and carried out HTS for a small-library LOPAC against DOT1L. After hit confirmation and profiling, we found that suramin inhibited DOT1L, NSD2, and PRMT4 with IC₅₀ values at a low μM range.

The authors studied the pharmacokinetics of levodopa (LD) with and without pramipexole (PPX) in men and postmenopausal women with PD. Patients on stable dose of carbidopa/LD were randomized to receive escalating doses of placebo or PPX over 7 weeks. LD and PPX pharmacokinetics were performed after a single test dose 25/100 of carbidopa/LD, before initiation of PPX or placebo, at 1.5 mg/d and 4.5 mg/d of PPX or placebo. Compared to men, women had greater LD bioavailability. PPX did not alter LD bioavailability, and PPX pharmacokinetics were equivalent in men and women.

Objectives: To determine the effect of reduced hepatic function on the pharmacokinetics of minoxidil. The pharmacokinetics of antipyrine, lorazepam, and indocyanine green were included as indicators of hepatic function., Methods: Eight mild cirrhotics and eight healthy subjects received antipyrine (po), lorazepam (IV), indocyanine green (IV) and minoxidil 5 mg (po). Blood and urine were sampled for up to 72 h after each drug, and drug concentrations were measured by validated HPLC methods. Blood pressure and heart rate were measured for safety., Results: For unchanged minoxidil, the serum elimination rate constant was significantly smaller and mean residence time was significantly longer in cirrhotic patients. Urinary elimination rate constant for minoxidil glucuronide was significantly smaller and fraction of dose excreted in urine was significantly higher in cirrhotic patients. Antipyrine elimination was significantly slower for cirrhotic patients. No differences were observed in lorazepam pharmacokinetic parameters., Conclusion: Pharmacokinetic analysis suggests a longer dosage interval may be appropriate in patients with hepatic impairment. In the absence of multiple-dose minoxidil pharmacodynamic studies in this population, minoxidil should be used as in other populations: begin with a modest dose, and adjust the dose based on clinical response.

The potential pharmacokinetic drug-drug interaction between delavirdine, a nonnucleoside analogue reverse transcriptase inhibitor, and indinavir, an inhibitor of HIV protease, was evaluated in healthy volunteers. Subjects received a single 800-mg dose of indinavir sulfate on day 1 (baseline). Delavirdine mesylate 400 mg was administered three times daily on days 2 through 10. On day 9, a single 400-mg dose and on day 10 a single 600-mg dose of indinavir were given along with morning doses of delavirdine. Pharmacokinetic evaluations of indinavir were made on days 1, 9, and 10, and of delavirdine on days 8, 9, and 10. Fourteen healthy male volunteers completed the study. Single doses of indinavir had no clinically important effects on the pharmacokinetics of delavirdine. Mean indinavir Cmax values for the 400-mg and 600-mg doses administered concomitantly with delavirdine were dose proportionally lower than that observed following the 800-mg dose administered alone. Mean Tmax values were similar and ranged from 1.0 +/- 0.3/hour for indinavir 800 mg administered alone to 1.3 +/- 0.4/hour for indinavir 600 mg administered with delavirdine. These results indicate that delavirdine had no clinically important effect on the rate of indinavir absorption. In contrast, the mean indinavir AUC0-infinity, value following the 400-mg dose administered with delavirdine was only 14% lower than the baseline value determined for the 800-mg indinavir dose (25,400 +/- 6960 nM hour versus 29,600 +/- 7920 nM hour), and the mean indinavir AUC0-infinity value for the 600-mg indinavir dose administered with delavirdine (42,700 +/- 9800 nM hour) was 44% greater than the baseline value. All differences among mean AUC0-infinity values were statistically significant. Mean indinavir half-life values were slightly longer when indinavir was given in a dose with delavirdine than when indinavir was administered alone. These results suggest that delavirdine inhibits metabolism of indinavir and support the possibility of a reduction in the magnitude or frequency of indinavir dosage when given in combination with delavirdine.

Objective: Oral contraceptive (OC) steroids alter the disposition of numerous drugs, including corticosteroids. We investigated the pharmacokinetics and pharmacodynamics of methylprednisolone., Methods: Twelve women (six women used OC steroids and six women did not) received intravenous methylprednisolone (0.6 mg/kg ideal body weight). Methylprednisolone disposition was assessed from plasma concentrations. Pharmacodynamic parameters measured were plasma cortisol, whole blood histamine (reflecting basophils), and blood helper T lymphocytes., Results: Methylprednisolone clearance was significantly decreased in the women who used OC steroids (0.298 versus 0.447 L/hr/kg), resulting in a longer elimination half-life (2.20 versus 1.72 hours). With use of indirect response models, significant differences were observed with the cortisol and basophil responses. A larger value for the concentration that inhibits the zero-order production rate by 50% (0.37 versus 0.11 ng/ml) was observed in the women who used OC steroids for suppression of cortisol secretion, indicating less sensitivity to the suppressive effects of methylprednisolone. Greater net suppression of basophils was observed in the users of OC steroids (area under the response curve, 694 versus 401 ng x hr/ml). No differences were observed for helper T-cell responses., Conclusion: OC steroids appear to inhibit methylprednisolone metabolism. However, mixed changes in several responses occur, indicating that women can probably receive similar doses of methylprednisolone irrespective of OC steroid use.

The pharmacokinetics of methylprednisolone were evaluated in 29 healthy volunteers after multiple intravenous doses of methylprednisolone sodium succinate or the novel prodrug, methylprednisolone suleptanate. Subjects were assigned randomly to one of four treatment groups (40, 100, 250, or 500 mg) and then randomly assigned to receive either the sodium succinate or suleptanate prodrugs. Doses were administered every 6 hours for 48 hours. Plasma and urine were assayed for methylprednisolone and unchanged prodrug using HPLC methods. Methylprednisolone pharmacokinetics exhibited both a dose and time dependency, which was similar for administration of both prodrugs. After first-dose administration, mean clearance increased from 19.5 L/hr for 40-mg doses to 27.7 L/hr after 500-mg doses of the sodium succinate ester, and from 20.1 to 31.7 L/hr after the suleptanate ester. After multiple dosing, mean clearance values increased from 31.1 to 44.7 L/hr for sodium succinate dosing, and from 31.5 to 46.0 L/hr for suleptanate dosing. Apparent systemic clearance values determined after multiple dosing were 1.5- to 1.8-fold greater than corresponding first-dose values. No dependence on time was apparent for any prodrug pharmacokinetic parameter. These data suggest that the dose dependency of methylprednisolone pharmacokinetics is related to dose-dependent prodrug hydrolysis, whereas the time dependence possibly reflects auto-induction of methylprednisolone metabolism. Based on comparison of methylprednisolone pharmacokinetic parameters derived for each prodrug, methylprednisolone suleptanate resulted in a faster and slightly more efficient conversion to methylprednisolone than methylprednisolone sodium succinate.

The influence of age on the pharmacokinetics of cefpodoxime was evaluated in 12 elderly (ages 65-85 years) and 12 weight- and sex-matched young (ages 20-33 years) subjects, each of whom received two cefpodoxime proxetil 200-mg tablets every 12 hours for 14.5 days. Serial blood samples and urine were collected after the first dose on day 1, after the morning dose on day 8, and after the last (morning) dose on day 15. Plasma and urine samples were assayed for cefpodoxime concentrations using HPLC methods. Within each age group, mean pharmacokinetic parameters determined on day 1 were similar to corresponding values on days 8 and 15, indicating that cefpodoxime does not accumulate after twice-daily dosing of cefpodoxime proxetil. Based on this result, parameters were pooled across days in each age group. No significant differences were observed between healthy and elderly volunteers in area under the plasma concentration-time curve for the 12-hour dosing interval, peak plasma concentration, or time to peak concentration. Mean urinary excretion and renal clearance of cefpodoxime were significantly lower in elderly subjects. Differences in renal clearance were attributed to the corresponding age-related reduction that was noted in creatinine clearance values, whereas the lower urinary excretion of cefpodoxime probably reflected slightly reduced systemic drug absorption in the elderly. Differences in these parameters between groups were less than 30%, and were unlikely to be of clinical importance. The data indicate that dose adjustment of cefpodoxime in elderly subjects having normal (age-adjusted) creatinine clearance values is not required.

The cutaneous bioavailability of topical 2% minoxidil solution was verified in live hairless mice. Minoxidil and propylene glycol deposition on the skin surface, epidermis and dermis from the single-dose in vivo study were compared with the results from previous in vitro studies. A distinct difference is apparent in the epidermis where the in vitro values are 11-22 times higher than the in vivo values for minoxidil and 8-16 times higher for propylene glycol. The differences were not as great in the dermis. Percutaneous absorption of the drug appeared to be a very small fraction of the applied dose. Similarly shaped stratum corneum and plasma concentration profiles and the relatively constant dermal profiles of minoxidil and propylene glycol open the possibility of transappendageal routes being involved in percutaneous absorption. The greater amount of drug and vehicle found in the dermis from in vitro studies can be explained by the absence of dermal clearance. The overestimation in the amount of drug found in the epidermis in vitro may also be attributable to poor dermal clearance. On the whole, the study raises questions about the use of in vitro tissue dispositions for bioavailability assessment and bioequivalence demonstration.

Relationships are drawn between the extent of topical delivery of test compounds in solution and the period of residence of their formulation on the skin. The studies were performed using in vitro diffusion cell techniques and a test formulation containing 2% 3H-minoxidil dissolved in 60% ethanol, 20% water and 20% 14C-propylene glycol. The permeation of propylene glycol was effectively halted upon cleansing the skin surface; the skin had very little reservoir capacity for this substance. However, the rate of delivery of minoxidil was only slowed but not stopped upon cleansing. The suggestion here is that a reservoir of minoxidil is formed in the skin which is capable of sustaining an appreciable input of drug even after the skin's surface is scrupulously cleaned. Assay of epidermal concentrations of these species not only confirms the existence of the minoxidil reservoir but also shows that the degree of its tissue concentration is proportional to the time of residence of the formulation on the skin surface. Reapplication of blank vehicle to the cleansed surface had little to no effect on the permeation of the minoxidil and was similarly without effect on that of propylene glycol. While it comes as no surprise that formulation residence time is an important variable in topical delivery, this study demonstrates the complexities of quantitative dependencies of delivery on residence time.

The pharmacokinetics and selected pharmacodynamic responses to methylprednisolone were investigated in six men and six premenopausal women after a dose of 0.6 mg/kg ideal body weight. Women (luteal phase) exhibited a greater methylprednisolone clearance (0.45 versus 0.29 L/hr/kg) and shorter elimination half-life (1.7 versus 2.6 hours) than men. The volume of distribution of methylprednisolone was similar when normalized for ideal body weight. Pharmacodynamic models were used to examine the methylprednisolone suppressive effects on cortisol secretion and basophil and helper T lymphocyte trafficking. A significantly smaller 50% inhibitory concentration (IC50) value (0.1 versus 1.7 ng/ml) was seen in the women for suppression of cortisol secretion, indicating increased sensitivity. However, the area under the concentration-time curve of effect was similar for both groups. The IC50 values for effects of methylprednisolone on basophil trafficking related to estradiol concentrations in a log-linear fashion in women, with increased sensitivity found at higher estradiol concentrations. Men displayed a greater 24-hour net suppression in blood basophil numbers, but no difference was observed in net cortisol and helper T lymphocyte suppression between the sexes. These findings suggest that methylprednisolone dosages should be based on ideal body weight. Although women are more sensitive to methylprednisolone as measured by cortisol suppression, they eliminate the drug more quickly, generally producing a similar net response.

We have studied the pharmacokinetics and haemodynamic effects of nicorandil after a 12-h infusion. Nicorandil is a mixed vasodilator combining the actions of a nitrate and a potassium channel opener. Nicorandil was infused for 12 h in 21 healthy volunteers at rates of 0.05, 0.10, and 0.20 microgram.kg-1.min-1 using a placebo controlled, crossover design. Systemic blood pressure, heart rate, electrocardiographic (ECG) intervals, and cardiac output (impedance cardiography) were measured supine and standing. Dose-related, steady-state plasma nicorandil concentrations occurred within 3 to 4 h. Nicorandil's pharmacokinetics were linear with dose. Four 0.20 microgram.kg-1.min-1 nicorandil infusions were terminated early primarily because of moderate or severe headaches. There were no safety concerns (ECG intervals, laboratory assays). Blood pressure fell versus placebo only in the standing position and heart rate increased slightly (not significant). That is, standing blood pressure in the 6 to 12 h interval fell from baseline 8.0*/6.8, 1.6/5.1, and 9.8*/7.9* mmHg (systolic/diastolic, * = P < 0.05 versus placebo) at 0.05, 0.10, and 0.20 micrograms.kg-1.min-1 respectively. Cardiac output increased slightly above placebo at lower doses. Haemodynamic changes correlated poorly with plasma nicorandil concentrations. Similar total doses were less well-tolerated when extended over 12 h. We saw no evidence of pharmacodynamic tolerance to nicorandil within 12 h.

We have studied the effects of intravenous nicorandil, a mixed arterial and venous vasodilator, in 48 healthy volunteers. Nicorandil (20, 28, 39, 54, 74, 103, 144, or 200 micrograms.kg-1) or placebo were given over 5 min to subjects supine (16 subjects, 2 doses) or sitting (32 subjects, 1 dose) in a single-blind crossover design. Electrocardiographic intervals, blood pressure, and heart rate were measured before and for 8 h after dosing. Blood and urine safety laboratory studies were also performed before and after dosing. All intravenous infusions of nicorandil and placebo were well tolerated and there were no clinically important safety concerns. The most frequent adverse event after nicorandil was headache (24 events by 19 subjects), although its occurrence was not strictly dose related. One subject experienced transient symptomatic hypotension (144 micrograms.kg-1). Mean plasma nicorandil concentrations were dose-related and fell with a half-life of 0.7 to 1.2 h. Systemic clearance and volume of distribution tended to decrease as dose increased. Sitting subjects showed marginally lower (< 20%) systemic clearances and larger values of Cmax and AUC. Nicorandil produced dose-related reductions in blood pressure, with consistent statistically significant differences from placebo after the 144 and 200 micrograms.kg-1 doses. The falls in blood pressure were greater for diastolic pressure and in this supine position. At 200 micrograms.kg-1, the mean falls in systolic/diastolic pressures (mm Hg) during the first hour were 10.9/14.7 supine and 6.1/9.1 sitting; systolic pressure returned to baseline after 8 h and diastolic pressure after 4 h.(ABSTRACT TRUNCATED AT 250 WORDS)

The disposition of minoxidil and propylene glycol from topical solutions was measured by using an in vitro mass balance technique. The experimental approach included assessment of the following compartments of the skin and the diffusion cell as a function of time: (1) donor compartment; (2) hairless mouse skin surface, epidermis, and dermis; and (3) receiver compartment. Excellent mass balance was achieved for minoxidil at three doses. However, the recovery of propylene glycol depended on both application volume and time. The experiment involving the evaporation of propylene glycol and water from the propylene glycol:ethanol:water (20:60:20, v/v) mixture, which was placed in the well of a tissue culture plate at room temperature and 37 degrees C, substantiated the loss of vehicles to the air. When a thin application of 20 microL/cm2 was used, 60% of the propylene glycol was unaccounted for after 16 h. The evaporation of propylene glycol concentrated the solution to supersaturation, precipitated out the drug, and then stabilized the thermodynamic activity of the drug in the vehicle. The amount of formulation applied influences the rate of concentration and, thus, the time at which minoxidil precipitates. The precipitation limits the amount of minoxidil that can be absorbed and leads to poor percutaneous absorption of drug from the formulation.

The absorption of flurbiprofen through human buccal mucosa was studied after 25 mL of a 2.5-mg/mL solution (pH 4) of the drug in a cosolvent mixture (ethanol 95%:glycerin:propylene glycol:0.3 M sodium acetate buffer, 10:40:30:20) was held and circulated in the mouth for 5 min in a "buccal absorption test." The results were compared with those obtained after oral administration of 25 mL of a solution (pH 7) of sodium flurbiprofen having the same concentration. Twelve subjects participated in the crossover study. After the buccal treatment, mean Cmax and Tmax values were 0.751 micrograms/mL and 41 min, respectively. Average Cmax and Tmax values after the oral treatment were 10.8 micrograms/mL and 32 min, respectively. Mean dose-corrected AUCs were 0.0854 and 0.811 (micrograms.h/mL)/mg for the buccal and the oral treatments, respectively. The absorption kinetics after the buccal treatment were evaluated using the Exact Loo-Riegelman Method (ELRM). Buccal plasma flurbiprofen concentration-time data for 11 subjects were very well fitted by reconstructed curves using ka and the lag-time obtained from ELRM analysis of the buccal data and the disposition parameters obtained from the oral data. These results strongly support the concept of intrasubject constancy of flurbiprofen disposition parameters. Analysis, by ELRM, of the plasma concentration-time data obtained after the buccal treatment indicated first-order absorption, with a mean ka value of 3.9 +/- 2.2 h-1. This value was significantly different (0.05 greater than p greater than 0.02) from the ka after oral treatment (7.89 +/- 5.2 h-1), obtained from the triexponential fitting of the oral plasma data.(ABSTRACT TRUNCATED AT 250 WORDS)

Twenty-two healthy male volunteers completed a four-way, multiple-dose, randomized crossover study to determine the relationship between contact time of applied drug on the scalp and minoxidil absorption from a 2% topical solution. One milliliter of solution was applied twice daily over 150 cm2 of bald scalp to each subject for 6 days. Unabsorbed drug was washed off the scalp after 1, 2, 4, and 11.5 h of contact time in each of four treatments. Cumulative urinary excretion profiles within steady-state, 12-h dosing intervals were well described by straight lines for all treatments, indicating that systemic minoxidil elimination was rate controlled by constant, zero-order percutaneous drug absorption. The extent of minoxidil absorption, expressed as steady-state urinary excretion of unchanged minoxidil, minoxidil glucuronide, or the sum of these, increased in a disproportionate manner with increase in contact time of drug on the scalp. Relative to the amount absorbed after a contact time of 11.5 h, absorption was approximately 50% complete by 1 h and greater than 75% complete by 4 h. This suggests that minoxidil absorption from the vehicle into skin occurs rapidly relative to diffusion through skin. The rate of minoxidil absorption from vehicle into skin was characterized as nonlinear, whereas minoxidil excretion into urine was rate controlled by diffusion from one or more components of the skin which apparently serve as a reservoir, or depot, for minoxidil.

Nineteen healthy male volunteers completed a three-way, randomized, crossover study to determine the effect of the synthetic retinoid, tretinoin, on percutaneous absorption of minoxidil. Subjects received, for 20 days, twice-daily administrations of 1 ml of an aqueous 2% topical minoxidil solution either alone, with once-daily applications of a 0.05% tretinoin cream, or with once-daily applications of a vehicle control cream. When minoxidil was coadministered with tretinoin cream, minoxidil absorption was increased nearly threefold, compared with a 1.3-fold increase in absorption observed with coadministration of vehicle control cream. Transepidermal water loss measurements, which are sensitive to changes in stratum corneum function, were also significantly increased with tretinoin. No treatment-related changes in stratum corneum thickness were observed on the basis of skin biopsy analysis. The findings indicate that percutaneous minoxidil absorption is enhanced by tretinoin as a result of increased stratum corneum permeability.

To evaluate the influence of multiple dose propranolol administration on the single dose pharmacokinetics of quinapril and its active metabolite, quinaprilat, a drug-drug interaction study was performed in ten healthy volunteers. Each subject received a single 20 mg quinapril oral dose on Days 1 and 16 of the study. Oral propranolol doses of 40 mg BID were initiated on Day 3, titrated gradually to 80 mg TID by Day 10, and continued at 80 mg TID through Day 17. Comparable mean quinapril pharmacokinetic parameter values as well as comparable mean quinaprilat pharmacokinetic parameter values determined following quinapril administered alone and following quinapril administered with propranolol, indicate that propranolol does not alter the single dose pharmacokinetics of quinapril or quinaprilat.

A randomized two-way crossover study was conducted in 12 healthy volunteers to assess the effect of food on the pharmacokinetics of quinapril (CI-906) and its active metabolite, CI-928, after quinapril dosing. Forty-milligram oral quinapril doses were administered in a fasted or a fed state with a one-week washout period between treatments. No significant treatment differences were observed in quinapril and CI-928 values for maximum plasma concentration, area under the plasma concentration-time curve, or percentage of dose excreted in the urine. Small but significant increases of less than 0.5 hour in quinapril and CI-928 tmax values were observed after consumption of food. The pharmacokinetic profiles of quinapril and CI-928 were not significantly altered by the administration of food.

Infusions of intravenous gamma-globulin (IVGG) are an effective, nontoxic therapy for chronic idiopathic thrombocytopenic purpura (ITP) that would be more widely accepted if the therapeutic agent were not so expensive. The costs and outcomes of managing such children with splenectomy and IVGG were modeled with Markov processes. Children unresponsive to one treatment were considered to have received the alternative. The model accounted for spontaneous remissions, therapeutic responses, traumatic events, episodes of sepsis, and operative deaths. For a 10-year-old child with chronic ITP, the strategy of initial treatment with splenectomy had associated costs of $17,000 and a 97.9% ten-year survival rate, whereas the strategy of initial treatment with IVGG had associated costs of $21,000 but a 98.6% survival rate. Each additional life saved by employing the IVGG strategy cost $540,000, or $8,000 per year for a life expectancy of 70 years. Sensitivity analyses demonstrated that for older children the IVGG strategy continued to result in improved survival rates but was more costly than the splenectomy strategy. For younger children, the IVGG strategy dominated, with improved survival rates and lower costs.

A patient with chronic lymphocytic leukemia was found to have pneumonitis caused by a simultaneous Pneumocystis carinii and Legionella pneumophila infection. Although both microorganisms frequently cause pulmonary infections in immunocompromised patients, co-infection has not been reported. This patient responded to antimicrobial therapy, but superinfection with Candida albicans led to his death. As there are numerous infective and noninfective causes of pneumonia in such patients, this case illustrates that the identification of a single etiologic agent does not obviate the search for other potential causes.

For single bolus administration, intermittent bolus administrations to steady-state, a single dose as a zero order input, intermittent zero order inputs to steady-state, and continuous zero order input to steady-state, and for both simple Michaelis-Menten elimination and parallel Michaelis-Menten and first order elimination, the appropriate equations are given for the areas, AUC 0-oo or AUC 0-tau, steady-state concentrations, and clearances. Some 20 new equations have been derived. For the case of first order input and Michaelis-Menten elimination, no solution is given but the effect of input rate on systemic availability is reported following some numerical integrations. The effect of slow input in reducing systemic bioavailability when Michaelis-Menten elimination kinetics are operative is stressed and the implications of this in the field of sustained-release medication mentioned.

Three simple linear and three simple non-linear pharmacokinetic models are presented which incorporate the reversible metabolism that occurs between prednisone and prednisolone. Under steady-state conditions it is possible to not only distinguish between the linear and non-linear models but also to determine which particular model of each group applies to a given set of data. While the non-linear conversion of prednisolone to prednisone is important in explaining the pharmacokinetics of prednisone in all three non-linear models, the same is not true for prednisolone.

The potential effect of cimetidine on the pharmacokinetic profiles of quinapril and its active metabolite CI-928 was evaluated in eight healthy volunteers. Each subject received a single 40-mg quinapril dose on days 1 and 12 and cimetidine 300 mg four times daily on days 8 through 13. Serial blood and urine samples were collected for assay of quinapril and CI-928 concentrations. No statistically significant differences were observed in quinapril or CI-928 Cmax, tmax, AUC(0-infinity), beta, or percent of dose excreted in urine values for quinapril administered alone and in combination with cimetidine. Therefore, multiple-dose cimetidine administration does not influence the single-dose pharmacokinetics of quinapril and its active metabolite, CI-928, in healthy volunteers.

Intravenous zero order infusions were administered to New Zealand white rabbits. In a pilot study using one rabbit, prednisone and prednisolone clearance values increased with increase in either prednisone or prednisolone infusion rates. In the second study, prednisone was infused until both prednisone and prednisolone achieved steady-state concentrations. In the third study, prednisone was infused until only prednisone achieved steady-state concentrations. The results of the three experiments support the use of a non-linear reversible metabolism model to describe the pharmacokinetic relationship between prednisone and prednisolone.

The protein binding characteristics of prednisone and prednisolone were determined in human and rabbit plasma and in a 4.7 per cent human serum albumin (HSA) solution. The influence of prednisolone on prednisone binding in human plasma was also examined. Prednisolone exhibited nonlinear binding and prednisone linear binding characteristics in both human and rabbit plasma. Prednisone binding was not influenced by the presence of prednisolone. Prednisone binding to HSA was linear but to a degree substantially lower than observed in human plasma, suggesting the possibility that prednisone binds to other proteins in human plasma. The results support the hypothesis that the protein binding characteristics of prednisone and prednisolone do not explain the reported nonlinear pharmacokinetics of prednisone.

The two-compartment model of Rowland et al., (2) has been extended by replacing first order elimination with Michaelis-Menten elimination kinetics. All of the equations for steady-state concentrations and clearances for zero order (constant rate) input orally (into compartment #2) and intravenously (into compartment #1) are derived and reported. The steady-state concentration in compartment #1, following intravenous administration, is shown to be a nonlinear function of maximal velocity of metabolism, Vm, the Michaelis constant, Km, and liver blood flow, Q; and, following oral administration is dependent only upon Vm and Km and is independent of Q. However, oral bioavailability is a function of Vm, Km, and Q. The model allows physiologic pharmacokinetic interpretation of both linear and nonlinear data; and, together with simple modification of the model, can explain much observed pharmacokinetic data to date particularly for first-pass drugs. Future articles in the series will be concerned with single doses, evaluation of literature data in terms of the model, application of the theory in toxicology and in clinical pharmacokinetics and therapeutics.

A randomized, four-way cross-over study was conducted in eight healthy male volunteers to determine the relative and absolute bioavailability of prednisone (PN) and prednisolone (PL). PN and PL were administered as single, oral 10-mg tablet doses and as 10-mg zero-order 0.5-hour intravenous infusions. Comparable mean PN and PL maximum plasma concentrations (Cmax), times for Cmax, areas under the plasma concentration-time curves (AUC), and apparent elimination rate constants between tablet treatments demonstrated that PN and PL tablets were bioequivalent. Absolute bioavailability (F) determinations based on plasma PL concentrations were independent of which IV treatment was used as reference and indicated complete systemic availability of PL from both PN and PL tablets. However, F based on plasma PN data was contradictory. Using IV PN as reference, approximately 70% systemic availability was observed from both tablets, whereas using IV PL as reference, systemic availability was greater than unity. PN and PL are model compounds that exemplify the difficulties involved in accurately determining the relative and absolute bioavailability of substances that undergo reversible metabolism.