Monte carlo simulation of an X-ray pixel beam microirradiation system.

Monte Carlo simulations are used in the development of a nanotechnology-based multi-pixel beam array small animal microirradiation system. The microirradiation system uses carbon nanotube field emission technology to generate arrays of individually controllable X-ray pixel beams that electronically form irregular irradiation fields having intensity and temporal modulation without any mechanical motion. The microirradiation system, once developed, will be incorporated with the micro-CT system already developed that is based on the same nanotechnology to form an integrated image-guided and intensity-modulated microirradiation system for high-temporal-resolution small animal research. Prospective microirradiation designs were evaluated based on dosimetry calculated using EGSnrc-based Monte Carlo simulations. Design aspects studied included X-ray anode design, collimator design, and dosimetric considerations such as beam energy, dose rate, inhomogeneity correction, and the microirradiation treatment planning strategies. The dosimetric properties of beam energies between 80-400 kVp with varying filtration were studied, producing a pixel beam dose rate per current of 0.35-13 Gy per min per mA at the microirradiation isocenter. Using opposing multi-pixel-beam array pairs reduces the dose inhomogeneity between adjacent pixel beams to negligible levels near the isocenter and 20% near the mouse surface.

Sensitivity of large-field electron beams to variations in a Monte Carlo accelerator model.

Adjustments made to Monte Carlo models during the commissioning of the simulation should be physically realistic and correspond to actual machine characteristics. Large electron fields, with the jaws fully open and the applicator removed, are sensitive to important source and geometry parameters and may provide the most accurate beam models, including those collimated by an applicator. We report on the results of a comprehensive Monte Carlo sensitivity study documenting the response of these large fields to changes in the configuration of a Siemens Primus linear accelerator. The study was performed for 6, 9 12, 15, 18 and 21 MeV configurations, and included variations of thickness, position and lateral alignment of all treatment head components. Variations of electron beam characteristics were also included in the study. Results were classified by their impact on central-axis depth dose distributions, including the bremsstrahlung tail, and on beam profiles near D(max) and in the bremsstrahlung region. Low-energy results show an increased sensitivity to electron beam properties. High-energy bremsstrahlung profiles are shown to be useful in determining misalignments between the beam axis and mechanical isocentre. For all energies, the alignment of the secondary scattering foil and monitor chamber are shown to be critical for correctly modelling beam asymmetries. The results suggest a methodology for commissioning of electron beams using Monte Carlo treatment head simulation.

Phencyclidine exposure and the developing mouse: behavioral teratological implications.

The effects of perinatal exposure to phencyclidine (PCP) on the reflex development of the offspring of mothers given PCP during gestation and/or lactation were determined. ICR Swiss mice received daily injections of either PCP (5, 10, 20 mg/kg, PO) or saline during gestation and/or lactation. Thus, four groups of animals were studied; those exposed only prenatally, postnatally, both pre- and postnatally, or control. After birth, these offspring were observed daily until weaning for the appearance of certain reflexes, using a modification of the Fox battery. There was a delay in the disappearance of the cross extensor reflex and delayed appearance of reflexes, such as walking, crawling, vibrissal placement and vibrissal stroking in the offspring of PCP-treated mothers. Treated animals also showed slower righting times than control animals. Growth rate was decreased in PCP-exposed animals beginning at 3 days of age and continuing through 15 days of age. These results indicate that PCP exposure during gestation or nursing adversely affects the development of behavioral reflexes in mice and suggest that regular observation of reflex ontogeny in neonates may be a sensitive indicator of behavioral teratology.

Phencyclidine: its transfer across the placenta as well as into breast milk.

Phencyclidine (PCP) is a dangerous and unpredictable drug which is widely abused among young people. Acute placental transfer of this drug was studied in pregnant rabbits and mice where it was shown to cross the placenta readily. Rabbit fetal levels of radioactivity reached their peak 2 hours after parenteral administration of PCP to the doe. In the mouse, where actual PCP levels were determined, there was a tenfold higher concentration of PCP in fetal tissue than in maternal blood. In lactating mice, the drug was found to cross rapidly into breast milk where it reached concentrations which were 10 times that of plasma. As PCP may be teratogenic and has been shown to be harmful to the infant during the postnatal period, those treating pregnant women should be aware to these possible routes of exposure for the developing infant and should counsel their patients accordingly.

N-Acetyltransferases of rat liver and blood: substrate specificities.

The substrate specificities of N-acetyltransferases (NAT) were investigated by measuring Vmax and Km values with p-aminobenzoic acid (I), p-aminobenzamide (II), p-amino-benzamidopyridine (III), 2-(p-aminobenzamido)-4,6-dimethylpyrimidine (VI), and the corresponding p-aminobenzenesulfonamides (VII, VIII and XI) using at rat liver and blood preparations. With liver NAT, II, III and VI had lower Km and higher Vmax values than did their corresponding sulfonyl analogs (VII, VIII, XI). III was extraordinarily active (Vmax 854 nmol/mg protein/h); in contrast, II gave a Vmax of 22.4. Sulfadiazine (IX) and sulfamerazine (X) were acetylated at a very slow rate. The activities of the blood enzymes on these compounds were very different. The Vmax values obtained with blood NAT for II, VI and VII were sharply decreased. Surprisingly, acetylation of III, VIII, IX and X could not be detected. In contrast to liver, the blood NAT gave lower values of both Vmax and Km for the S analog of II and a much higher Km for I. While p-aminobenzoic acid was the best substrate for blood NAT, substitution of the amido nitrogen of p-aminobenzamide with an aromatic substituent enhanced the substrate potential for liver NAT, III may be useful as a substrate for the rapid classification of slow and fast acetylators.

Pharmacokinetics of cephalosporin antibiotics: protein-binding considerations.

The therapeutic activity of antibiotics depends on several factors including absorption, elimination kinetics, distribution in the body, minimal inhibitory concentrations (MIC), stability against enzymes, and plasma-protein binding. Some of these factors are interrelated, for example, the extent of protein binding of an antibiotic influences its elimination kinetics, distribution into tissues, MIC, and antibacterial activity. To evaluate the potential efficacy of an antibiotic, it is important to know the extent of its binding to plasma proteins especially since the protein-bound fraction of the antibiotic is devoid of antibacterial activity. Cephalosporins are a new class of broad-spectrum antibiotics that bind to plasma proteins in different degrees. Reported values for protein binding range from 6% for cephradine to 92% for cefazolin. The effects of protein binding of some of the commonly used cephalosporins on antibacterial activity and several pharmacokinetic parameters are discussed in this communication.

Metabolism of the (+)-, (+/-)-, and (-)-enantiomers of alpha-methylfluorene-2-acetic acid (cicloprofen) in rats.

1. After oral or intraperitoneal administration of (+/-)-[14C]cicloprofen to rats, the peak plasma concentrations of radioactivity and the areas under the plasma concentration/time curves did not increase proportionally with dose; total urinary and faecal excretions of radioactivity did increase with dose, suggesting saturation of plasma protein binding of drug and faster elimination of unbound drug at higher doses. 2. [14C]Cicloprofen and its metabolites were eliminated mainly via biliary excretion. Ratios of faecal to urinary excretion ranged from 2 to 3 and depended on dose administered. 3. Rats with cannulated bile ducts excreted the drug almost exclusively in bile, whereas intact rats excreted up to 32% of the dose in urine in 6 days, suggesting that [14C]cicloprofen or its metabolites or both undergo extensive enterohepatic recirculation in the rats. 4. The major metabolites of [14C]cicloprofen excreted in urine or bile were the 7-hydroxy, 9-hydroxy-, 7,9-dihydroxy-, and 9-hydroxy-9-methoxy-derivatives and their glucuronide or sulphate conjugates. 5. The (+)-enantiomer of [14C]cicloprofen was hydroxylated and excreted by rats at a faster rate than its (-)-antipode; no qualitative stereoselective metabolism of the individual enantiomers of [14C]cicloprofen was observed.

Inversion of optical configuration of alpha-methylfluorene-2-acetic acid (cicloprofen) in rats and monkeys.

A simple and sensitive radiometric method to determine the individual enantiomers of cicloprofen has been developed. 14C-Cicloprofen was converted to its L-leucine diastereoisomers, which were separated by thin-layer chromatography and quantified by measuring the radioactivity in the area corresponding to each individual diastereoisomer. This technique has also been used to measure the enantiomers of unlabeled cicloprofen by condensing with 14C-labeled L-leucine. By using the radiometric method, a unique biotransformation process, the inversion of the (-)-enantiomer of alpha-methylfluorene-2-acetic acid to its (+)-enantiomer, has been demonstrated in the rat and monkey. The rate of (-)- to (+)-inversion was found to be faster in the rat than in the monkey. After single or repeated oral adminstration of the racemic modification or the (-)-enantiomer of cicloprofen to both species, the ratio of (+)- to (-)-enantiomers of cicloprofen in plasma, urine, or bile increased with time. At 5, 22, and 48 hr after oral administration of a single 50-mg/kg dose of the (-)-enantiomer, 14C-cicloprofen in rat plasma contained 20, 50, and 79%, respectively, of the (+)-enantiomer. After receiving the same dose of (-)-enantiomer, monkey plasma contained 16.5% and 32% of (+)-enantiomer at 8 and 24 hr, respectively. After oral administration of a single 50-mg/kg dose of the (+)-enantiomer of 14C-cicloprofen to rats and monkeys, the percentage of (-)-enantiomer in plasma varied from 2 to 15%. Since the administered (+)-enantiomer contained 4% of (-)-enantiomer and the (+)-enantiomer was excreted at a faster rate than its (-)-antipode by rats or monkeys, it is not known whether an occasional small percentage increase of (-)-enantiomer in plasma resulted from the (+)-to-(-) inversion, or from faster elimination of the (+)-enantiomer. Nevertheless, if (+)-to-(-) inversion does occur in these two species, the rate is much slower than for the (-)-to-(+) inversion.

Fluorometric determination of cephradine in plasma.

A fluorometric method was developed for the determination of cephradine in plasma. A fluorescent product is formed when samples of deproteinized plasma containing cephradine are heated for 3 hr at 100 degrees and pH 1. The fluorescence is determined in sodium hydroxide solution (pH 13.5) at excitation and emission wavelengths of 350 and 445 nm, respectively. Only 0.1 ml of plasma is required, and concentrations of cephradine as small as 0.1 mug/ml may be determined. In plasma samples from a dog taken over a 10-hr period after an intramuscular injection of 250 mg of cephradine, essentially similar concentrations of cephradine were obtained by the fluorometric method and a standard microbiological bioassay.

Release and elimination of 14C-fluphenazine enanthate and decanoate esters administered in sesame oil to dogs.

The rates of release of 14C-fluphenazine enanthate and 14C-fluphenazine decanoate were compared in two groups of five male dogs. Each dog was given a single dose (2 mg/kg im) of either the enanthate or decanoate ester in sesame oil. The times required to attain maximum concentrations of radioactivity in plasma were 3.8 +/- 0.5 days (+/-SE) for the enanthate ester and 10.6 +/- 1.1 days for the decanoate ester (p less than 0.001); maximum concentrations of radioactivity in the plasma at these times were 16.7 +/- 1.1 and 11.1 +/- 1.2 ng/ml, respectively (p less than 0.01). However, 35 days after dosing, the concentrations of radioactivity in plasma were greater for the decanoate ester than for the enanthate ester. The times required for 50% of the dose to be excreted in the urine and feces were 7.8 +/- 0.5 days for the enanthate ester and 22.6 +/- 4.4 days for the decanoate ester (p less than 0.05). The total amounts excreted in 35 days were 85.4 +/- 1.8 and 68.8 +/- 6.6% of the dose for the enanthate and decanoate esters, respectively; the average half-times for the rates of release of radioactivity from depot and body, as calculated from the data for total excretion, were 5.55 days for the enanthate ester and 15.4 days for the decanoate ester. Thirty-five days after dosing, the amount of the dose present in the injection site was 4.6 +/- 1.6% for the enanthate ester and 18.6 +/- 5.7% for the decanoate ester. Two groups of six dogs each were protected against the emetic effects of apomorphine more than twice as long by the decanoate ester than by the enanthate ester after the subcutaneous administration of single 8-mg/kg doses of either drug in sesame oil (p less than 0.05). Based on measurements of total radioactivity, it was concluded that the decanoate ester was released from the depot at less than one-half the rate of the enanthate ester.

Absorption and excretion of a new antidepressive (SQ 10,996) in humans.

A single oral dose of 10 mg of SQ 10,996-14C was absorbed slowly by 3 normal male volunteers, with peak plasma concentrations achieved 6 hr after ingestion; the plasma half-life was about 38.5 hr. On average, 82.3 +/- 3.5% of the radioactivity present in the 2 hr plasma sample was bound to plasma proteins. These volunteers excreted an average of 31 and 52% of the dose in the urine and feces, respectively. All subjects excreted minor amounts of 14CO2 in the expired air. No unchanged SQ 10,996-14C was found in the urine. Three unidentified metabolites were excreted in urine. SQ 10,996-14C was excreted in the feces only as unchanged drug, suggesting that the drug is incompletely absorbed. The volunteers tolerated the drug well and experienced no adverse effects.

Stereospecific inversion of l-alpha-methylfluorene-2-acetic acid to its d-enantiomer in the dog.

1. After administration of dl-alpha-methylfluorene-2-acetic acid to dogs, the optical rotation of the drug in blood increased with time. Of the total drug in blood, the d-enantiomer increased from 61 to 80% between 3 and 24 h after administration; by 384 h it was 100%. 2. Both l- and d-enantiomers had plasma half-lives and excretion characteristics similar to those of the dl-racemic mixture, indicating that the increase in the proportion of the d-enantiomer was not due to more rapid excretion of the l-enantiomer. 3. Studies of optical rotation and circular dichroism demonstrated that the l-enantiomer was converted to the d-enantiomer in the blood of the dog, but the d-enantiomer remained unchanged. After administration of the l-enantiomer, the d-enantiomer increased from 26 to 71% of the total drug in blood between 0-3 and 2 days after administration; 14 days after dosing, almost all of the drug was present as the d-enantiomer. 4. Isomerization of l-alpha-methylfluorene-2-acetic acid to its d-enantiomer also occurs in rat, monkey and man.

Metabolism of a benzothiazine compound (SQ 11,579) by the intact rat, isolated perfused rat liver, and rat-liver microsomes.

1. The metabolic dispositions of a benzothiazine compound (SQ 11,579) by the intact rat, isolated perfused rat liver, and rat-liver microsomes have been investigated, and the results compared. 2. The drug was well absorbed after oral administration to rats and was widely distributed in all tissues, which, with the exception of brain, had higher concentrations of the drug, its metabolites, or both, than did plasma. 3. Metabolism by rat-liver microsomes included N-oxidation, N-demethylation, S-oxidation and aryl hydroxylation. Metabolites hydroxylated in the aromatic ring were excreted only in bile, both by the isolated perfused rat livers and by anaesthetized bile-duct-cannulated rats. 4. Liver perfusion of the benzothiazine or its monodesmethyl analogue (V) resulted in temporary cessation of the flow of perfusate through the organ. The benzothiazine sulphoxide (IV) had only a slight effect on the flow of liver perfusate, but IV followed by I caused the flow of perfusate to cease.

Autoradiographic and tissue distribution studies on a nitrovinylfuran derivative (SQ 18506 14C) in S. mansoni infected mice.

Autoradiographic and tissue distribution studies of SQ 18506 14C were carried out on 13 Swiss albino mice. Infection was done by I.P. route with 50 to 60 cercaria of S. mansoni (puerto Rican strain) per animal. The safe single i.v. dose of a solution containing one mg of SQ 18506 dissolved in 0.06 ml dimethylsulfoxide was 0.003 ml/g b.w. of mice. Schistosome autoradiograms were clearly demonstrated one day after that dose injected in each mouse 50 days post-infection. However, hepatic autoradiograms were visible 14 days after 2 doses of our drug to each infected mouse. The higher the concentration of SQ 18506 14C in the culture medium the darker were the schistosomes in the autoradiograms and the greater their total d.p.m/mg. Tissue distribution studies after 2 1/2 doses of our drug/mouse revealed that schistosome total d.p.m/mg dry-weight was 50 times more than that of its liver. The latter d.p.m/g wet-weight was slightly higher than that of one ml of mouse blood.

Metabolism of 8-(methylthio)cyclic 3',5'-adenosine monophosphate by rats and dogs after oral or intravenous dosing and in vitro by subcellular preparations of dog liver.

8-(Methylthio-14C or -35S)cyclic 3',5'-adenosine monophosphate (I) was given intravenously to rats (5 mg/kg) and orally and intravenously to dogs (0.25, 2.5, or 50 mg/kg). Oral doses were absorbed well but slowly. Plasma half-lives in dogs were about 3 hr after oral or intravenous doses of 0.25 or 2.5 mg/kg and ranged from 5 to 12 hr after oral or intravenous doseas of 50 mg/kg. Plasma glucose and insulin concentrations in dogs were increased by oral or intravenous doses of the compound. Regardless of the route, excretion of radioactivity by rats and dogs at all doses was chiefly in the urine (74-87% of the dose); the remainder was excreted in the feces or bile. Compound I was rapidly distributed to most tissues of dogs but entered the brain and certain portions of the eye slowly and to a limited extent. Urine and plasma of dogs and urine of rats contained I, 8-(methylthio)adenosine, and at least two other unidentified metabolites. Compound I and cyclic 3',5'-adenosine monophosphate were metabolized in vitro by the soluble fraction of dog liver to form 8-(methylthio)adenosine-5'-monophosphate and adenosine-5'-monophosphate, respectively. These compounds were further converted to 8-(methylthio)adenosine and adenosine, respectively. Compound I was metabolized in vitro more slowly than cyclic 3',5'-adenosine monophosphate.