Disposition of growth hormone-releasing peptide (SK&F 110679) in rat and dog following intravenous or subcutaneous administration.

The disposition of growth hormone releasing peptide (SK&F 110679) has been studied in male Sprague-Dawley rats and in male and female beagle dogs following intravenous (iv) and subcutaneous (sc) administration. Mass balance/excretion of [3H]SK&F 110679 was assessed in bile duct-exteriorized rats from which radiolabeled biliary and urinary excreta were quantified and characterized. [3H]SK&F 110679 was excreted, predominantly in the bile, and to a large extent as intact peptide following either iv or sc administration. Although the extent of biliary excretion of radiolabel was similar following iv or sc administration (60-70% of the dose), the rate was significantly higher following iv administration. Using a specific plasma HPLC/fluorescence assay, the iv and sc pharmacokinetics of SK&F 110679 were investigated in both species. Following iv bolus administration, biphasic plasma concentration-time profiles were observed, and the initial phases were characterized by 2-4 min half-lives. Systemic plasma clearance was 27 ml/min/kg in the rat (0.4 mg/kg dose) and 17 ml/min/kg in the dog (0.5 mg/kg dose). High sc bioavailability (89-103%) was observed in both species; an apparent terminal half-life of 1 hr likely reflected slow absorption from the injection site.

Pharmacokinetic evaluation of drug interactions with anti-human immunodeficiency virus drugs. V. Effect of soluble CD4 on 2',3'-dideoxycytidine kinetics in monkeys.

This study was conducted to determine if soluble CD4 (ST4) altered the pharmacokinetics of 2',3'-dideoxycytidine (ddC) in nonhuman primates. Each of six monkeys received 5 mg/kg of ddC iv in the absence and presence of two different iv regimens of ST4. The ST4 regimens produced steady-state plasma concentrations of 10.3 micrograms/ml (N = 3) and 22.2 micrograms/ml (N = 3) for 30 min following ddC administration. Pharmacokinetic parameters for ddC and ST4 were calculated based on plasma and urine concentrations of ddC and plasma concentrations of ST4. Following combined ddC and ST4 administration, in both the low- and high-dose ST4 groups, plasma concentration-time profile of ddC were similar for each monkey, and no statistical differences were observed in the pharmacokinetic parameters compared with those obtained when ddC was given alone. Complete urinary excretion data for ddC was obtained in 3 of the 6 animals studied. At the low ST4 dose, one animal had a reduced renal clearance of ddC, whereas at the high ST4 dose two animals recorded an increased renal clearance of ddC. ST4 plasma concentrations were comparable to in vitro concentrations of antiviral activity, with pharmacokinetic parameters similar to those reported previously. The kinetic information provides a basis for rational dosage design for combination chemotherapeutic regimens of ddC and ST4 in human immunodeficiency virus infection.

Evaluation of the acute hemodynamic effects and pharmacokinetics of coronary thrombolysis produced by intravenous tissue-type plasminogen activator in the anesthetized dog.

The thrombolytic dose-response effectiveness and pharmacokinetics of tissue-type plasminogen activator (rt-PA) was evaluated in anesthetized, open-chest dogs instrumented for the measurement of systemic hemodynamics. Intracoronary thrombi were formed by injecting thrombin (100 U) and CaCl2 (50 microM) into a cannulated, isolated segment of the left anterior descending coronary artery (LAD). Coronary blood flow was measured by placing an electromagnetic flow probe proximal to the LAD thrombus. Thirty minutes after formation of a stable LAD thrombus, intravenous infusion of rt-PA was given at rates of 0.5, 1, 2, 4, or 8 micrograms/kg/min (n = 8/dose) for 60-90 min, and the animals were followed for an additional 30 min. In vehicle-treated animals, residual thrombus wet weight, determined at the end of the experiment, was 30 +/- 4 mg (mean +/- SEM, n = 8) and spontaneous reperfusion did not occur. The rt-PA produced a dose-related increase in the number of animals reperfusing, a decrease in the time to reperfusion, and a decrease in residual thrombus weight, but had no effect on systemic hemodynamics. The increase in infusion rate from 0.5 to 4 micrograms/kg/min resulted in a linear increase in both the steady-state rt-PA plasma concentration and the area under the rt-PA plasma concentration versus time curve (n = 3-5 animals/dose); between the infusion rates of 4 and 8 microgram/kg/min there was a disproportionate increase in both these parameters that was due to a decrease in the total systemic clearance of rt-PA. The postdosing elimination half-life (t1/2 alpha) did not differ significantly at any dose of rt-PA, and the pooled half-life (t1/2 alpha) for all doses of rt-PA was 2.36 +/- 0.12 min (n = 19).(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-dependent pharmacokinetics of recombinant tissue-type plasminogen activator in anesthetized dogs following intravenous infusion.

The pharmacokinetics of SK&F recombinant two-chain tissue-type plasminogen activator (tPA) following intravenous (iv) infusion were characterized in anesthetized, open chested mongrel dogs in which artificial intracoronary thrombi were formed. SK&F tPA was infused at rates of 0.5, 1, 2, 4, and 8 micrograms/kg/min (N = 3 to 5 per dose) for 90 min, and arterial blood samples were withdrawn during and after infusion for determination of functionally active tPA concentrations using a modified and validated S-2251 chromogenic assay. At all doses studied, steady state active tPA plasma concentrations were achieved 10-20 min after the onset of infusion. Upon cessation of infusion, active tPA plasma concentrations declined rapidly with a t1/2 of 2-3 min. The active tPA plasma concentration at steady state (Css) and the area under the tPA plasma concentration-time curve (AUC) increased linearly with the dose in the range of 0.5-4 micrograms/kg/min. However, as the dose was increased 2-fold from 4 to 8 micrograms/kg/min, the AUC and the Css increased 2.5-fold. The systemic clearance ranged from 15-16 ml/min/kg at doses of 0.5-4 micrograms/kg/min, but decreased to 11.7 ml/min/kg at the 8 micrograms/kg/min dose. With exceptions in three dogs, the volume of distribution at steady state approached or slightly exceeded the blood volume. Plasma tPA antigen concentrations were also determined in the dogs receiving the 2 micrograms/kg/min dose. At steady state, active tPA accounted for 40-60% of the total tPA antigen. The postinfusion t1/2 of the tPA antigen was considerably longer (13.46 +/- 5.94 min) than that of active tPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Positional isotope exchange and kinetic experiments with Escherichia coli guanosine-5'-monophosphate synthetase.

The kinetic mechanism of Escherichia coli guanosine-5'-monophosphate synthetase has been determined by utilizing initial velocity kinetic patterns and positional isotope exchange experiments. The initial velocity patterns of MgATP, XMP, and either NH3 or glutamine (as nitrogen source) were consistent with the ordered addition of MgATP followed by XMP and then NH3. The enzyme catalyzes the exchange of 18O from the beta-nonbridge positions of [beta,beta,beta gamma,gamma,gamma,gamma-18O6]ATP into the alpha beta-bridge position only in the presence of XMP and Mg2+. The exchange reaction did not require NH3. The isotope exchange reaction increased as the XMP concentration increased and then decreased at saturating levels of XMP. These results also support the ordered addition of MgATP followed by XMP. GMP synthetase catalyzes the hydrolysis of ATP to AMP and PPi along with an ATP/PPi exchange reaction in the absence of NH3. These data taken together support a mechanism in which the initial step in the enzymatic reaction involves formation of an adenyl-XMP intermediate. Psicofuranine, an irreversible inhibitor of the enzyme, acts by preventing the release or further reaction of adenyl-XMP with H2O or NH3 but does not suppress the isotope exchange or ATP/PPi exchange reactions. GMP synthetase has also been shown to require a free divalent cation for full activity. When Ca2+ replaces Mg2+ in the reaction, the positional isotope exchange reaction is enhanced but the reaction with NH3 to form GMP is greatly suppressed.