In-vitro measurements

The proposed pathway for the biosynthesis of the avermectins (Fig. 3) has been described in a review (23). Some of the details are yet to be elucidated, although the steps, in general, are based on firm evidence from four types of studies incorporation of labeled precursors, conversion of putative intermediates by producing strains and blocked mutants, in vitro measurement of biosynthetic enzymes, and studies with enzyme inhibitors. The biosynthesis of the oleandrose units was elucidated from studies using and labeled glucose, which indicated a direct conversion of glucose to... 

Looking at the foregoing results overall, the rates of loss in vivo are related to the rates of metabolism in vitro, measured or estimated. As with the OC insecticides, problems of persistence are associated with compounds that are not readily metabolized, for example, 2,2, 4,4, 5,5 -HCB in the foregoing examples. For further discussion of the dependence of elimination of lipophilic xenobiotics on metabolism, see Walker (1981). 

Accuracy of in vivo and in vitro measurements of americium is determined through the use of standard, certified radioactive sources with known concentrations of americium. The primary source of certified americium standards is the National Institute of Standards and Technology (NIST). Standard solutions are available for241 Am (SRM 4322, 40 Bq/g [1.1 nCi/g]) and 243Am (SRM 4332, 40 Bq/g [1.1 nCi/g]). Standard Reference Materials for human lung (SRM 4351) and human liver (SRM 4352) are also available from NIST. 

The in vitro measurements of permeability by the cultured-cell or PAMPA model underestimate true membrane permeability, because of the UWL, which ranges in thickness from 1500 to 2500 pm. The corresponding in vivo value is 30-100 pm in the GIT and nil in the BBB (Table 7.22). The consequence of this is that highly permeable molecules are (aqueous) diffusion limited in the in vitro assays, whereas the membrane-limited permeation is operative in the in vivo case. Correcting the in vitro data for the UWL effect is important for both GIT and BBB absorption modeling. 

T. van Vliet, F. van Schaik, W. H. Schreurs, and H. van den Berg, In vitro measurement of beta-carotene cleavage activity Methodological considerations and the effect of other carotenoids on beta-carotene cleavage, Int. J. Vitam. Nutr. Res. 66 (1996) 77-85. 

Shutes A, Der CJ (2005) Real-time in vitro measurements of GTP hydrolysis. Methods 37 183-189... 

Takadama, H., Hashimoto, M., Mizuno, M. and Kokubo, T. (2004) Round-robin test of SBF for in vitro measurement of apatite-forming ability of synthetic materials. Phosphorus Research Bulletin, 17, 119-25. 

Grass GM, Sweetana SA (1988) In vitro measurement of gastrointestinal tissue permeability using a new diffusion cell. Pharm Res 5 372-376... 

Measurement of tensile or shear stress is the most commonly used in vitro method to determine bioadhesion. All in vitro measurements provide a rank order of bioadhesive strength for a series of candidate polymers. Measurement of tensile strength involves quantitating the force required to break the adhesive bond between the test polymer... 

Szczepaniak performed in vitro measurements using Intralipid as a model for IMCL and soybean oil for EMCL, confirming the theoretical assumptions and in vivo experiments of Boesch. 

Figure 2.5. Setup for in vitro measurement of blood-brain barrier permeability with a co-culture of bovine brain microvascular endothelial cells (BBMEC) and an astro ioma cell line, C6. The BBMEC are grown on top of a filter insert. The C6 cells are either grown on the opposite side of the filter or on the bottom of the wells. Transport across the BBMEC monolayer is measured by adding the test substance to the upper chamber and sampling from the lower chamber. The tightness of the monolayer is also characterized by the transendothelial electrical resistance (TEER). Courtesy of T. Abbruscato.

These values are obtained from in vitro enzyme experiments. From the previous relahonship between in vitro pharmacology measurements and free drug concentra-hons and those outHned here, it is reasonable to assume that cHnical dose size can be calculated from simple in vitro measurements. 

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