Selected noble-gas partition coefficients

Argon pharmacokinetics: measurements in pigs and analysis in humans using a physiologically based pharmacokinetics model

The primary objective of this study was to investigate the pharmacokinetics of inhaled argon in young pigs using mechanical ventilation. Also a physiologically based model of argon pharmacokinetics (PBPK) is validated with human data for xenon from the literature and the new data from juvenile pigs. The inherent difficulty in performing pharmacokinetics studies of argon makes the use of the PBPK model especially relevant. The model is used to investigate argon pharmacokinetics for adult and neonate applications. Juvenile pigs (n = 4) were anesthetized, submitted to endotracheal intubation, and mechanical ventilation using a conventional ventilator. Argon inhalation was achieved by switching the animal from the first mechanical ventilator (with air/oxygen) to a second one that was supplied with 75% argon and 25% oxygen from premixed gas cylinders. This administration yielded blood samples that were analyzed using a quadrupole based technique for determining argon concentration. The range of blood:gas partition coefficient corresponding to the average measured Cmax of 190-872 μM is 0.005-0.022. Based on the average curve, T1/2= 75 seconds. The PBPK is shown to be in general agreement with the experimental data in pigs. Inhaled argon administration exhibited an on-off nature such that AUC was proportional to administration time. Confidence in the PBPK model and the remarkably robust and stable on-off nature of argon pharmacokinetics, notwithstanding intersubject variability and comorbidity, suggests that inhaled argon could readily be applied to any treatment regime.

Pharmacokinetic analysis of the chronic administration of the inert gases Xe and Ar using a physiological based model

Background

New gas therapies using inert gases such as xenon and argon are being studied, which would require chronically administered repeating doses. The pharmacokinetics of this type of administration has not been addressed in the literature.

Methods

A physiologically based pharmacokinetics (PBPK) model for humans, pigs, mice, and rats has been developed to investigate the unique aspects of the chronic administration of inert gas therapies. The absorption, distribution, metabolism and excretion (ADME) models are as follows: absorption in all compartments is assumed to be perfusion limited, no metabolism of the gases occurs, and excretion is only the reverse process of absorption through the lungs and exhaled.

Results

The model has shown that there can be a residual dose, equivalent to constant administration, for chronic repeated dosing of xenon in humans. However, this is not necessarily the case for small animals used in pre-clinical studies.

Conclusions

The use of standard pharmacokinetics parameters such as area under the curve would be more appropriate to assess the delivered dose of chronic gas administration than the gas concentration in the delivery system that is typically reported in the scientific literature because species and gas differences can result in very different delivered doses.

A generic biokinetic model for noble gases with application to radon

To facilitate the estimation of radiation doses from intake of radionuclides, the International Commission on Radiological Protection (ICRP) publishes dose coefficients (dose per unit intake) based on reference biokinetic and dosimetric models. The ICRP generally has not provided biokinetic models or dose coefficients for intake of noble gases, but plans to provide such information for (222)Rn and other important radioisotopes of noble gases in a forthcoming series of reports on occupational intake of radionuclides (OIR). This paper proposes a generic biokinetic model framework for noble gases and develops parameter values for radon. The framework is tailored to applications in radiation protection and is consistent with a physiologically based biokinetic modelling scheme adopted for the OIR series. Parameter values for a noble gas are based largely on a blood flow model and physical laws governing transfer of a non-reactive and soluble gas between materials. Model predictions for radon are shown to be consistent with results of controlled studies of its biokinetics in human subjects.

Age-dependent changes in oxygen tension, radiation dose and sensitivity within normal and diseased coronary arteries-Part A: dose from radon and thoron

PURPOSE

There is mounting evidence that a significant fraction of radiation-induced mortality and years-life lost are non-cancerous in nature. This study quantifies the radon dose to the coronary artery walls, especially the intimal layer, vulnerable to the development of atherosclerosis, and associated cardiovascular disease (CVD). Two accompanying papers determine the oxygen levels (Part B) in coronary arteries and the oxygen effect for radon and other exposures (Part C).

MATERIALS AND METHODS

The alpha-radiation dose to coronary artery walls was calculated from the proportion of inhaled radon ((222)Rn), thoron ((220)Rn) and their short-lived progeny, which was not deposited in the lung and passed into blood. Age- and gender-dependent morphology and composition for the wall layers of coronary arteries were developed from published data for a normal population and also for individuals with cardiovascular disease. The alpha particle dose to the coronary artery walls was evaluated taking account the diffusion of radon from blood and the solubility of radon-gas in tissues.

RESULTS

Diseased arteries exhibited a moderate increase in the solubility of lipophylic radon (190%) in arteries with 88% luminal narrowing, as the high Rn solubility in fat was partially offset by the lower solubility in calcium deposits. The average worldwide dose rate to the diseased intimal layer from (222)Rn and its short-lived progeny was estimated to be as high as 68 muSv y(-1) per 40 Bq m(-3) in air, whereas the corresponding dose rate from (220)Rn per 0.3 Bq m(-3) in air was <or=0.1% in comparison. Gender had little influence on the dose.

CONCLUSION

The Rn dose to the coronary arteries is significant, but has a large uncertainty due to poor knowledge of Rn and its progeny concentrations in the body.

Exploring lung function with hyperpolarized129Xe nuclear magnetic resonance

With the use of polarization-transfer pulse sequences and hyperpolarized (129)Xe NMR, gas exchange in the lung can be measured quantitatively. However, harnessing the inherently high sensitivity of this technique as a tool for exploring lung function requires a fundamental understanding of the xenon gas-exchange and diffusion processes in the lung, and how these may differ between healthy and pathological conditions. Toward this goal, we employed NMR spectroscopy and imaging techniques in animal models to investigate the dependence of the relative xenon gas exchange rate on the inflation level of the lung and the tissue density. The spectroscopic results indicate that gas exchange occurs on a time scale of milliseconds, with an average effective diffusion constant of about 3.3 x 10(-6)cm(2)/s in the lung parenchyma. Polarization-transfer imaging pulse sequences, which were optimized based on the spectroscopic results, detected regionally increased gas-exchange rates in the lung, indicative of increased tissue density secondary to gravitational compression. By exploiting the gas-exchange process in the lung to encode physiologic parameters, these methods may be extended to noninvasive regional assessments of lung-tissue density and the alveolar surface-to-volume ratio, and allow lung pathology to be detected at an earlier stage than is currently possible.

An improved synthesis of the inert, diffusible blood-flow tracer, [18F]fluoromethane

An improved procedure for producing [18F]fluoromethane ([18F]FM) in batches of several hundred mCi is reported. 18F prepared by the 18O (p, n) 18F reaction in a H2(18)O target is trapped on a small column of Dowex 1 (x 10) resin to allow recovery of H2(18)O. One new feature is elution of 18F- from the column with 3 mL of CH3CN containing 67 microL 1.5 M aqueous (Bu)4N+ OH-, after residual H2(18)O has been removed with dry CH3CN. This 18F- solution reacts with CH3I in the presence of Ag2O directly to give [18F]FM, which is swept out of the reaction vessel with a stream of air, from which CH3I and other vapors are removed with a C18 SEP-PAK at room temperature. Another new technique is trapping of [18F]FM on a second SEP-PAK cooled in ethanol/dry ice. After warming the SEP-PAK to room temperature, the trapped [18F]FM can be recovered with either H2O or air. The improvements speed the preparation and minimize hands-on operations. The product has no detectable radiochemical impurities, and a specific activity of greater than 1 Ci/mumol. Non-radioactive CH3CN, CH3I and CH3OH are present at less than 0.2 mumol per batch.

An in vivo technique for the measurement of bone blood flow in animals

A new technique to measure the in vivo clearance of 41Ar from the bone mineral matrix is demonstrated following fast neutron production of 41Ar in bone via the 44Ca(n, alpha) reaction at 14.1 MeV. At the end of irradiation, the 41Ar activity is assayed with a Ge(Li) detector where sequential gamma-ray spectra are taken. Following full-energy peak integration, background and dead time correction, the activity of 41Ar as a function of time is determined. Results indicated that the Ar washout from bone in rats using this technique was approximately 16 ml (100 ml min)-1 and in agreement with other measurement techniques. For sheep the bone perfusion in the tibia was approximately 1.9 +/- 0.2 ml (100 ml min)-1.