Precision of measurement of tissue concentrations by RLG

Simple methodology for ensuring the precision of measuring radioactivity at low concentrations in very small tissues using quantitative whole-body autoradiography

Quantitative whole-body autoradiography (QWBA) is largely used to evaluate tissue distribution of small molecule drugs. In QWBA, radioactivity is measured as the intensity obtained from the autoradiogram. It is known that lower intensity per a region of interest (ROI) or smaller size of ROI increases the variability of intensity. In fact, as some tissues are very small (e.g., the choroidea), ensuring reliability on the intensity for measuring radioactivity in these tissues is difficult in case of under- or over-estimation of radioactivity concentration owing to their variation of low radioactivity intensity of ROI. We thus analyzed the relationships between the size, intensity, and precision of ROI to determine the statistically significant lower limit of quantification (LLOQ) in very small tissues. To investigate the difference in correlation between the radiation source (commercial planar radiation standard [com-ST] and self-made radiation standard [self-ST] consisting of radioactive compounds and matrices), apparatus, or setting environment of the apparatus, correlation analysis was conducted under various conditions. Our results revealed that LLOQ can be calculated by simply using the correlation equation because a common relationship was observed between self-ST, which is used in QWBA, and com-ST. This methodology was thus considered valuable for ensuring LLOQ determination in QWBA.

Effect of immune complex formation on the distribution of a novel antibody to the ovarian tumor antigen CA125

3A5 is a novel antibody that binds repeated epitopes within CA125, an ovarian tumor antigen that is shed into the circulation. Binding to shed antigen may limit the effectiveness of therapeutic antibodies because of unproductive immune complex (IC) formation and/or altered antibody distribution. To evaluate this possibility, we characterized the impact of shed CA125 on the in vivo distribution of 3A5. In vitro, 3A5 and CA125 were found to form ICs in a concentration-dependent manner. This phenomenon was then evaluated in vivo using quantitative whole-body autoradiography to assess the tissue distribution of (125)I-3A5 in an orthotopic OVCAR-3 tumor mouse model at different stages of tumor burden. Low doses of 3A5 (75 μg/kg) and pathophysiological levels of shed CA125 led to the formation of ICs in vivo that were rapidly distributed to the liver. Under these conditions, increased clearance of 3A5 from normal tissues was observed in mice bearing CA125-expressing tumors. Of importance, despite IC formation, 3A5 uptake by tumors was sustained over time. At a therapeutically relevant dose of 3A5 (3.5 mg/kg), IC formation was undetectable and distribution to normal tissues followed that of blood. In contrast, increased levels of radioactivity were observed in the tumors. These data demonstrate that CA125 and 3A5 do form ICs in vivo and that the liver is involved in their uptake. However, at therapeutic doses of 3A5 and clinically relevant CA125 levels, IC formation consumes only a minor fraction of 3A5, and tumor targeting seems to be unaffected.

Methods determining phosphor imaging limits of quantitation in whole-body autoradiography rodent tissue distribution studies affect predictions of 14C human dosimetry

INTRODUCTION

Radioluminography, or phosphor imaging, is often used in rodent quantitative whole-body autoradiography (QWBA) studies to determine the tissue distribution and pharmacokinetic (PK) parameters of new pharmaceutical entities in rodents. The rodent tissue pharmacokinetics information are then used to predict human radiation exposure to 14C or 3H during human radioisotope mass balance studies. The human dosimetry estimation can be biased by the method used to determine the lower limit of quantitation (LOQ) of the phosphor imager. A survey of autoradiographers revealed that at least five different methods are used to determine phosphor imager LOQ. The objective of this study is to compare and evaluate the human dosimetry estimates obtained by applying those five LOQ methods to a single set of WBA data.

METHODS

Five different phosphor imager LOQ determination methods were applied to a single set of QWBA rodent tissue distribution data to produce five tissue concentration time profiles. Tissue PK parameters were determined for each profile and subsequently used to calculate the 14C exposure for a proposed human 14C mass balance study.

RESULTS

A threefold difference was observed among the five predictions of human 14C exposure when the five different phosphor imager LOQ values were applied to the initial data set.

DISCUSSION

The method chosen to determine the phosphor imaging LOQ for QWBA rodent tissue distribution study could impact the human 14C exposure estimates. The end result may either under- or overestimate the 14C-tissue exposure in humans during radioisotope studies, depending on the method used to determine LOQ. We recommend two approaches to reduce the variations in the determination of rodent tissue distribution pharmacokinetics: (1) Set more sampling time points to cover the terminal phase to obtain more accurate t 1/2 and (2) use Method 3 or the small sized sampling tool of Method 5 for LOQ determinations because it is a balanced approach for both simplicity and accuracy.

Quantitative whole-body autoradiography in the pharmaceutical industry. Survey results on study design, methods, and regulatory compliance

INTRODUCTION

Quantitative whole-body autoradiography (QWBA) is a technique used to determine the tissue distribution of radiolabeled compounds in laboratory animals. This relatively new technique is quickly replacing wet-tissue dissection techniques, which, up to now, have been used by the pharmaceutical industry when performing tissue distribution studies to develop new drugs and to address regulatory compliance needs. In an effort to harmonize QWBA procedures across the pharmaceutical industry, the Society for Whole Body Autoradiography (SWBA) surveyed its membership to gain insight into the procedures and practices being used to perform tissue distribution studies conducted in support of drug development.

METHODS

The survey polled 29 respondents, who represent pharmaceutical companies in the United States, Europe, and Asia. Participants answered approximately 50 questions related to study design, applications, autoradiography methods, tissue quantitation, and regulatory compliance.

RESULTS

The survey revealed general consistencies and inconsistencies among the labs that responded. Consistencies were related to: isotope use and doses of radioactivity, number of animals per time point, exsanguination of animals, freezing methods, section thickness, tissue collection lists, section lyophilization, imaging technology, blood and calibration standards, tissues and sections sampled for quantitation, use of QWBA data for human dosimetry, and QWBA method validation. Inconsistencies were related to: number of time points used, euthanasia methods, carcass freezing time, microtome calibration, section thickness verification, sample collection, validation of commercial standards, use of background measurements during calibration, definition of limits of quantitation, reporting of extrapolated values, reexposure of section to determine low levels, computer system validation, definitions of raw data, audit trail documentation, studies performed under Good Laboratory Practices (GLP) vs. non-GLP conditions.

DISCUSSION

The survey indicated that most labs are now using QWBA to perform their tissue distribution studies and that these data have been submitted and accepted by regulatory authorities around the world. Procedures and practices involved in the design of these studies appear to vary somewhat. An important inconsistency found related to the number of time points used to determine the pharmacokinetic (PK) parameters for tissues, which may effect the reliability of these parameters for use in predicting human exposure to radioactivity during human radiolabeled studies. Survey results regarding QWBA methods indicated that there is a lot of consistency across surveyed labs; however, there are some inconsistent areas that raise regulatory compliance issues and these are related to the verification of section thickness, validation of commercial standards and their use in quantitation, definitions of limits of quantitation, and consideration of background measurements during quantitation. This survey provides autoradiographers, managers, and regulators with an important reference on the state-of-the art of QWBA and shows that the technique has gained wide acceptance across the pharmaceutical industry. However, it also shows that there are some key areas, such as inconsistencies in the procedures used for quantitation, that investigators may want to probe further to assure that the highest quality and most useful studies are performed.

Quantitative distribution studies in animals: cross-validation of radioluminography versus liquid-scintillation measurement

The results of a cross-validation of the radioluminography (RLG) and liquid scintillation counting (LSC) methods are presented. The methods for the determination of radioactivity concentrations were compared in 16 organs, after administration of (14)C-labeled substances to rats. LSC measurements of two kinds were used as reference methods for RLG: (1) quantitative determination of radioactivity after conventional dissection (interindividual comparison) and (2) quantitative determination of radioactivity in tissue punches taken from the whole-body sections after they had undergone RLG measurement (intraindividual comparison). Blood standards containing known concentrations were used for calibration. For statistical evaluation log-linear regression analysis of paired concentration values and organ-specific 95% confidence intervals of the log-transformed RLG/LSC concentration quotients were compared. For most organs, the slopes of the regression lines and the means of the concentration quotients were within the defined equivalence range of 0.80-1.25. Deviations were distinctly smaller in the intraindividual comparison. For some organs, however, it became clear that found concentrations were affected by self-absorption (RLG) and by differences in sample preparation (LSC). In conclusion, quantification with RLG is a reliable and reproducible method with comparable measurement precision and greater accuracy in respect of tissue localization, compared to LSC (dissection).

Quantitative whole-body radioluminography-future strategy for balance and tissue distribution studies

The present routine to conduct balance and/or tissue dissection distribution studies has now and then been questioned, because the way they are generally conducted does not produce information in proportion to the spending of animal and personnel resources. Usually only total radioactivity is measured and due considerations are not always taken to the metabolic fate of the label. In this study a different strategy is presented-integrating quantitative whole-body radioluminography and different chromatographic methods on extracts of tissue pieces punched from the whole-body sections. In addition to the saving in cost and time, the proposed integrated whole-body radioluminographic/metabolic profile protocol will provide (i) a detailed picture of the distribution of radioactivity at selected dose levels and time points in male, female, and pregnant animals; (ii) the time course of radioactivity in blood/plasma and tissues selected from the images (approximate half-life and AUC); (iii) accumulated urinary and fecal excretion of radioactivity and an estimate of the proportion of radioactive metabolites; (iv) tissue information about the proportion of parent drug versus metabolites of pieces punched from the whole-body sections; and (v) indications of possible tissue binding.