Perfusion studies

It should be remembered that with the possible exception of voltammetry when the monitoring electrode is sufficiently small to reach synapses, it is not the actual release of the NT that is being measured in perfusion studies. It is overflow. As discussed previously, most of any released NT is either physically restricted to the synapse or destroyed before it can diffuse away. 

There is much evidence (e.g. Cheramy, Leviel and Glowinski 1981) from both in vitro and in vivo perfusion studies that DA is released from the dendrites of DA neurons in both A9 and AlO even though those dendrites do not contain many vesicles compared with axon terminals. The release and changes in it may also be slower and longer than that at axon terminals and the synaptic arrangement between the releasing dendrites and postsynaptic target is not clear. DA receptors also appear to be on neurons other than dopamine ones and on the terminals of afferent inputs to A9 (and AlO). It seems that the activation of the DA neurons may partly be controlled by the effects of the dendritically released DA on such inputs. 

Schoener, EP and Elkins, DP (1984) Neuronal response to dopamine in rat neostriatum. A push-pull perfusion study. Neuropharmacology 25 611-616. 

In situ perfusion studies assess absorption as lumenal clearance or membrane permeability and provide for isolation of solute transport at the level of the intestinal tissue. Controlled input of drug concentration, perfusion pH, osmolality, composition, and flow rate combined with intestinal region selection allow for separation of aqueous resistance and water transport effects on solute tissue permeation. This system provides for solute sampling from GI lumenal and plasma (mesenteric and systemic) compartments. A sensitive assay can separate metabolic from transport contributions. 

Most of the perfusion studies have been carried out on the isolated liver since it is in this organ that the change of fat to carbohydrate presumably takes place. On the basis of some of these studies, carbohydrate formation from fat has been postulated.161-166 In such cases more carbohydrate has been present at the end of the perfusion experiment than could be accounted for on the basis of known carbohydrate precursors. In several cases, a decrease in the fat content of the liver was accompanied by a concomitant increase in carbohydrate. However, Gregg166 has been... 

These refinements in our knowledge of brain penetration and CNS activity of drugs feature prominently in a major medicinal review of the blood-brain barrier [14]. In vivo perfusion studies on the rate of brain uptake of several non-steroidal anti-inflammatory drugs in rats with increasing concentration of albumin in the perfusate clearly demonstrate the effect of plasma protein binding on the rate (in addition to the extent at steady-state) of brain uptake [15]. 

The permeability of the drug substance can be determined by different approaches such as pharmacokinetic studies in humans (fraction absorbed or mass balance studies) or intestinal permeability studies (in vivo intestinal perfusion studies in humans or suitable animal models or in vitro permeation studies using excised intestinal tissue or epithelial cell culture monolayers like CaCo-2 cell line). In order to avoid misclassification of a drug subject to efflux transporters such as P-glycoprotein, functional expression of such proteins should be investigated. Low- and high-permeability model... 

Alyautdin RN, Tezikov EB, Ramge P, Kharkevich DA, Begley DJ, Kreuter J (1998) Significant entry of tubocurarine into the brain of rats by adsorption to polysorbate 80-coated polybutylcyanoacrylate nanoparticles an in situ brain perfusion study. J Microencapsul 15 67-74. 

Lennernas H (2000) Animal Perfusion Studies. In JB Dressman, H Lennernas (Eds.), Oral Drug Absorption Prediction and Assessment. New York, NY, Dekker, Inc. pp 73-98. 

Figure 5.1 Experimental setup for ex vivo nasal perfusion studies [4] (Reproduced with kind permission from Elsevier B.V.).

J. Kraemer, J. Klein, A. Lubetsky, and G. Koren. Perfusion studies of glyburide transfer across the human placenta Implications for fetal safety. Am J Obstet Gynecol. 195 270-274 (2006). 

R. Boskovic, D.S. Feig, L. Derewlany, B. Knie, G. Portnoi, and G. Koren. Transfer of insulin lispro across the human placenta In vitro perfusion studies. Diabetes Care. 26 1390-1394 (2003). 

Albumin-based air-filled microbubbles were reasonably stable on storage,but not very stable in the bloodstream. Air was rapidly lost and dissolved in the surrounding blood after administration to the patients. As a result, attempts to use this agent for perfusion studies in the clinical practice were not very successful, and it was limited mostly to opacification of the left ventricle and endocardial border defineation. 

The results of the intestinal perfusion studies of Lyons et al. (48) cannot be explained on the basis of the barrier disruption hypothesis. The barrier disruption hypothesis would predict an increase in the overall transport of radioactivity in the presence of potentiators this was not observed by Lyons et al. ( ). The changes in the ratio of histamine to its metabolites obtained in the experiments of Lyons et al. ( ) would not be predicted by the barrier disruption hypothesis either. 

Krypton-81m has been used to study cardiopulmonary systems from as early as 1970 (X,2). Since then other investigators have proposed its use for myocardial perfusion, (3,4, cerebral perfusion, C5, >), and venography, (.7) A Rb-81 /Kr-81m generator for pulmonary ventilation studies has been commercially available since 1980 from Medi-Physics, Inc. The present work will focus on the manufacture and testing of a Rb-81/Kr-81m generator suitable for liquid elution and use in perfusion studies. 

The Rb-82 generator permits serial studies in the same patient as often as every 10 minutes with 20-60 mCi of Rb-82 for rapid bolus intravenous infusion. Inherent in the administration of high levels of Rb-82 activity is the need for precise flow control from an automated system to deliver the desired amount of radioactivity. The development of the alumina column parameters and the elution protocol as well as the automated microprocessor system controller are presented here. Some of the details of this system have been discussed in earlier publications (15,21). Generator produced Rb-82 is used as a diffusible flow tracer in myocardial perfusion studies and as a nondiffusible tracer in brain studies to assess blood brain barrier permeability changes in patients with brain tumors or Alzheimer s type dementia. 

Figure 8. Myocardial perfusion studies with Rh-82 indicating areas of infarction.

The following methods can be used to determine the permeability of a drug substance from the gastrointestinal tract (1) in vivo intestinal perfusion studies in humans (2) in vivo or in situ intestinal perfusion studies using suitable animal models (3) in vitro permeation studies using excised human or animal intestinal tissues or (4) in vitro permeation studies across a monolayer of cultured epithelial cells. 

FIGURE 22.6 Permeability and permselectivity of vaginal and buccal epithelia in the rabbit, (a) Flux of 6-carboxyfluoroscein, a hydrophilic molecule, by in vitro perfusion studies steady-state flux ( xg/cm2/h x 106), (b) resistance (fl cm2 x 10 2), (c) thickness (p,m x 10-2), and (d) ratio of potassium transport number to chloride transport number, which is calculated from electrical measurements, used as indicative of the epithelium selectivity for positively charged molecules. (Modified from Sayani, A.P. and Chien, Y.W., Crit. Rev. Ther. Drug Carrier Syst. 13, 85, 1996.)... 

A series of 2 -(0-acyl) derivatives of 9-(2-hydroxyethoxymethyl)gua-nine (acyclovir) was synthesized by Shao and coworkers.63 64 The bioconversion kinetics of the prodrugs appeared to depend on both the polar and the steric properties of the acyl substituents. Rat nasal perfusion studies using the in situ perfusion technique showed no measurable loss of acyclovir from the perfusate. Also, the extent of nasal absorption appeared to depend on the lipophilicity of the prodrugs. All the prodrugs showed enhanced absorption. Branching of the acyl... 

In situ intestinal perfusion studies are typically done with live animals in which a perfusion loop has been inserted into the intestine (233,424). Depending on the experimental protocol, the system can offer a relatively unbiased view of... 

Fig. 3.4. Rates of Gly uptake from glycine and diglycine test solutions which are equivalent in Gly content (e.g. 20 mM Gly v. 10 mM Gly-Gly). The perfusion studies were done in the jejunums of five healthy human volunteers. (After Adibi, S. A. Kim, Y. S. Peptide absorption and hydrolysis. In Physiology of the gastrointestinal tract, ed. L. R. Johnson, 1981, with permission from Raven Press, New York.)...

Teng MM, Cheng HC, Kao YH et al (2001) MR perfusion studies of brain for patients with unilateral carotid stenosis or occlusion evaluation of maps of time to peak and percentage of baseline at peak . J Comput Assist Tomogr 25 121-125... 

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