Factors affecting nitroxide reduction in ascorbate solution and tissue homogenates.

Because of their paramagnetic properties, nitroxides are potentially useful as contrast agents in magnetic resonance imaging (MRI). They are reduced in vivo to their corresponding hydroxylamines which are nonparamagnetic and have no contrast enhancing property. Nitroxides with high resistance to reduction would be advantageous as pharmaceutical contrast enhancing agents. We show that in the presence of ascorbic acid and in tissue homogenates, the reduction is faster for piperidine than for pyrrolidine nitroxides and for positively-charged than for negatively-charged derivatives. The data also suggest that nitroxide reduction in tissue homogenates is mainly due to sulfhydryl groups on proteins and that endogenous ascorbic acid plays a relatively minor role.

Enhanced MRI of tumors utilizing a new nitroxyl spin label contrast agent.

Nitroxyl spin labels have been shown to be effective in vivo contrast agents for magnetic resonance imaging (MRI) of the central nervous system, myocardium, and urinary tract. A new pyrrolidine nitroxyl contrast agent (PCA) with better resistance to in vivo metabolic inactivation than previously tested agents was studied for its potential to enhance subcutaneous neoplasms in an animal model. Twenty-two contrast enhancement trials were performed on a total of 15 animals 4-6 weeks after implantation with human renal adenocarcinoma. Spin echo imaging was performed using a .35 T animal imager before and after intravenous administration of PCA in doses ranging from 0.5 to 3mM/kg. The intensity of tumor tissue in the images increased an average of 35% in animals receiving a dose of 3 mM/kg. The average enhancement with smaller doses was proportionately less. Tumor intensity reached a maximum within 15 min of injection. The average intensity difference between tumor and adjacent skeletal muscle more than doubled following administration of 3 mM/kg of PCA. Well-perfused tumor tissue was more intensely enhanced than adjacent poorly perfused and necrotic tissue.

Theoretical model for both saturable rate and extent of absorption: simulations of cefatrizine data.

A pharmacokinetic model incorporating saturable rate of absorption of the Michaelis-Menten type was recently developed to fit cefatrizine (CFZ) plasma concentrations with time following oral administration of 500-mg capsules to humans. This model (MM) was statistically superior to models incorporating either first-order or zero-order absorption. However, the MM model does not predict the reduction in extent of absorption with dose observed in vivo. In this study, a model is proposed in which a time constraint, delta t, is added to the MM model. This new model (MM-delta t) is tested with data following doses of 250, 500, and 1000 mg of CFZ. When delta t is set to 1.5 hr, the predicted relative changes with dose in bioavailability, F, peak plasma concentration, Cmax, the time at which the peak concentration occurs tmax, and the mean absorption time, MAT, are generally in good agreement with the experimental data. The time interval of 1.5 hr is compatible with passage by a limited region within the small intestine where drug is absorbed by a facilitated transport mechanism. Influence of each absorption model parameter (Vmax, Km, and delta t) on total area under the concentration versus time curve (AUC), F, Cmax, and tmax, is assessed by simulation. The MM-delta t model is able to summarize the nonlinerity observed in both rate and extent of absorption.

Pharmacokinetics of two prototype nitroxide spin labels for contrast enhancement in magnetic resonance imaging.

Two nitroxide spin labels called PCA and TES have been used experimentally as contrast enhancing agents in magnetic resonance imaging. Pharmacokinetic data for these nitroxides, after intravenous administration to three dogs at two dose levels (0.1 and 2.5 mmole/kg), are presented. In this dose range, the clearance (13 ml/ min-kg) and half-life (22 min) of PCA stayed almost constant while TES clearance decreased (28 to 12 ml/min-kg) and half-life increased (8 to 17 min) with dose. The urinary recovery, determined from the sum of the nitroxide and its corresponding hydroxylamine, was 85 to 90% for both PCA and TES. PCA was investigated in more detail because of its lower clearance value and its lack of dose-dependence. The major pathways of elimination are renal excretion and metabolic reduction to the corresponding hydroxylamine which is eliminated by renal excretion. We estimated renal clearance of both PCA (5.5 ml/ min-kg) and its hydroxylamine (3.7 ml/min-kg) to be close to glomerular filtration rate (4 ml/min-kg) in the dog, while the metabolic clearance (7.5 ml/min-kg) was slightly higher. Approximately 35 % of the administered dose of PCA was excreted unchanged and half was reduced in the body.

Dose-dependent bioavailability and metabolism of salicylamide in dogs.

The dose-dependent first-pass metabolism and pharmacokinetics of salicylamide (SAM) were studied at four dose levels in six dogs. Four minutes after each oral dose, a tracer dose of [14C]SAM was given i.v. to determine clearance and bioavailability. Over the dosage range studied, 5 to 40 mg/kg, bioavailability increased from 0.24 +/- 0.14 (mean +/- S.D.) to 0.76 +/- 0.20 (P less than .05). Clearance decreased from 3.4 +/- 1.0 to 0.60 +/- 0.11 liters/min (P less than .01) and half-life increased from 5.0 +/- 1.2 to 23.5 +/- 6.1 min (P less than .01). Measurement of SAM clearance to individual metabolites indicated that the sulfoconjugation and not the glucuronidation pathway was responsible for the dose-dependent effects observed. These effects occurred even at doses not expected to have caused significant depletion of body stores of inorganic sulfate; the plasma concentration of inorganic sulfate decreased by only a maximum of 13 and 26% after the 5- and 10-mg/kg SAM doses, respectively. When [14C]SAM was given alone in tracer amounts, clearance values greatly exceeded cardiac output. This suggests that SAM undergoes sulfation in organs other than the liver and intestinal wall.

Pharmacokinetics and metabolic fate of two nitroxides potentially useful as contrast agents for magnetic resonance imaging.

Paramagnetic nitroxyl-containing compounds have been useful as contrast agents in magnetic resonance imaging (MRI) experiments in animals. Preliminary information on the metabolic fate, pharmacokinetic behavior, stability in tissues, and chemical reduction of two prototypic nitroxides, PCA and TES, is presented. In the dog TES was eliminated more rapidly than PCA. More than 80 % of the dose of both nitroxides was recovered in urine within 6 hours. Nitroxides were reduced in vivo to their corresponding hydroxylamines. No other metabolite was observed. Measured reducing activity in tissue homogenates was greater in liver or kidney than in brain, lung or heart. In each tissue PCA was more stable than TES. PCA was also more resistant to reduction by ascorbic acid at physiologic pH. These preliminary results favor the use of PCA, a pyrrolidinyl nitroxide, over TES, a piperidinyl nitroxide, for MRI contrast enhancement.