Validation of procedures for quantitative whole-body autoradiography using digital imaging

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.

Whole-body tissue distribution of total radioactivity in rats after oral administration of [¹⁴C]-bilastine

This study evaluated the tissue distribution of total radioactivity in male albino, male pigmented, and time-mated female albino rats after oral administration of a single dose of [¹⁴C]-bilastine (20 mg/kg). Although only 1 animal was analyzed at each time point, there were apparent differences in bilastine distribution. Radioactivity was distributed to only a few tissues at low levels in male rats, whereas distribution was more extensive and at higher levels in female rats. This may be a simple sex-related difference. In each group and at each time point, concentrations of radioactivity were high in the liver and kidney, reflecting the role of these organs in the elimination process. In male albino rats, no radioactivity was measurable by 72 hours postdose. In male pigmented rats, only the eye and uveal tract had measurable levels of radioactivity at 24 hours. Measureable levels of radioactivity were retained in these tissues at the final sampling time point (336 hours postdose), indicating a degree of melanin-associated binding. In time-mated female rats, but not in albino or pigmented male rats, there was evidence of low-level passage of radioactivity across the placental barrier into fetal tissues as well as low-level transfer of radioactivity into the brain.

Impact of various factors on radioactivity distribution in different DBS papers

Background: Dried blood spot (DBS) sampling could potentially become the preferred blood collection technique in toxicological and clinical studies. Autoradiography was performed to study compound distribution within a dbs under different conditions using five papers, 31ETF, Grade 226, 903®, FTA® and FTA® Elute. Results: The results showed an uneven distribution in all papers with common distribution patterns regardless of compounds: decreased concentrations along the edge, the volcano effect in the middle and the speckle pattern in the center. Treated papers were more readily influenced by environmental factors. Conclusion: Autoradiography enables visualization of a compound’s distribution and can guide bioanalytical assay development by allowing convenient evaluation of factors, such as choice of paper, spotting volume, punch size, punch location, temperature and humidity.

Sustained polymeric delivery of gene silencing antisense ODNs, siRNA, DNAzymes and ribozymes: in vitro and in vivo studies

Small interfering RNA (siRNA), antisense oligonucleotides (ODNs), ribozymes and DNAzymes have emerged as sequence-specific inhibitors of gene expression that may have therapeutic potential in the treatment of a wide range of diseases. Due to their rapid degradation in vivo, the efficacy of naked gene silencing nucleic acids is relatively short lived. The entrapment of these nucleic acids within biodegradable sustained-release delivery systems may improve their stability and reduce the doses required for efficacy. In this study, we have evaluated the potential in vitro and in vivo use of biodegradable poly (D,L-lactide-co-glycolide) copolymer (PLGA) microspheres as sustained delivery devices for ODNs, ribozyme, siRNA and DNA enzymes. In addition, we investigated the release of ODN conjugates bearing 5'-end lipophilic groups. The in vitro sustained release profiles of microsphere-entrapped nucleic acids were dependent on variables such as the type of nucleic acid used, the nature of the lipophilic group, and whether the nucleic acid used was single or double stranded. For in vivo studies, whole body autoradiography was used to monitor the bio-distribution of either free tritium-labelled ODN or that entrapped within PLGA microspheres following subcutaneous administration in Balb-c mice. The majority of the radioactivity associated with free ODN was eliminated within 24 h whereas polymer-released ODN persisted in organs and at the site of administration even after seven days post-administration. Polymer microsphere released ODN exhibited a similar tissue and cellular tropism to the free ODN. Micro-autoradiography analyses of the liver and kidneys showed similar bio-distribution for polymer-released and free ODNs with the majority of radioactivity being concentrated in the proximal convoluted tubules of the kidney and in the Kupffer cells of the liver. These findings suggest that biodegradable PLGA microspheres offer a method for improving the in vivo sustained delivery of gene silencing nucleic acids, and hence are worthy of further investigation as delivery systems for these macromolecules.

Interaction of ritonavir on tissue distribution of a [(14)c]L-valinamide, a potent human immunodeficiency virus-1 protease inhibitor, in rats using quantitative whole-body autoradiography

N-[(3-fluorophenyl)methyl]glycyl-N-[3-[((3-aminophenyl)sulfonyl)- 2-(aminophenyl)amino]-(1S,2S)-2-hydroxy-1-(phenylmethyl)propyl]- 3-methyl-L-valinamide (DPC 681, DPC(1)) on oral coadministration with ritonavir (RTV) in rats caused a significant increase in systemic exposure to DPC. Following a single oral dose of [(14)C]DPC with and without RTV pretreatment in rats, and subsequent analysis of whole-body sections, prepared at 1 and 7 or 8 h postdose, using whole-body autoradiography showed an increase in radioactivity in tissues (e.g., brain, and testes) upon coadministration. The distribution of radioactivity in the brain parenchyma and ventricles was different, such that the concentration of radioactivity was greater in cerebrospinal fluid (CSF) than in central nervous system. Thus, the use of CSF concentration of the total radioactivity as a surrogate for brain penetration would result in an overestimation. DPC was determined to be metabolized prominently by rCYP3A4. The increased tissue exposure to DPC in rats could largely be attributed to inhibition of CYP3A1/2 by RTV. DPC was also a good substrate for P-glycoprotein (Pgp), with K(m) of 4 microM and V(max) of 13 pmol/min. The Pgp-mediated transport of DPC across Caco-2 cells was readily saturated at >or=10 microM and was inhibited significantly by RTV at 5 to 10 microM. The data above and the reported RTV concentrations suggested that both the Pgp and CYP3A4 inhibition by RTV may play a significant role in enhancing the systemic and tissue exposure to DPC in humans.

Biodistribution study of [(123)I] ADAM in mice: correlation with whole body autoradiography

Iodine-123 labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I] ADAM) has been suggested as a promising serotonin transporter (SERT) imaging agent. Much research has been accomplished, mainly focusing on the SERT binding sites in the central nervous system (CNS). However, the biodistribution of [(123)I] ADAM using whole body autoradiography (WBAR) has never been previously described, to the best of our knowledge. In this study, we assayed the biodistribution of [(123)I] ADAM in tissues/organs removed from mice, and measured their radioactivity with a scintillation counter (SC). The results showed that the liver has the highest uptake. On the other hand, the WBAR clearly demonstrated that [(123)I] ADAM was bound to SERT-rich sites including those in the brain stem, lung, adrenal glands and intestinal mucosa. This radiotracer also accumulated in the liver, kidney, and thyroid. The results from both methods were compared; each has its own complementary role in the biodistribution studies. The SC method revealed the total amount of radiotracer accumulation in each organ, and the WBAR demonstrated more anatomical details of the radiotracer's distribution. The whole body distribution results of the radioligand using both methods explore the usage of this novel radioligand for most possible SERT binding sites, not only in the CNS but also in the peripheral nervous system and neuroendocrine tissues. These findings suggest that [(123)I] ADAM is a potentially useful imaging agent for SERT.

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.

Dissection autoradiography: a screening technique using storage phosphor autoradiography to detect the biodistribution of radiolabelled compounds

INTRODUCTION

This study reports an alternative, rapid, whole body autoradiography technique which utilises storage-phosphor imaging technology. Conventionally, tissue or whole body sections have been used to examine the distribution of radiolabelled test compounds. However, the information acquired relates only to the sections examined, and the amount of radioactivity within the whole organ cannot be quantified. We have developed a rapid semi-quantitative technique that produces a concise visual representation of the distribution of the isotope throughout the entire animal: dissection autoradiography (DAR).

METHODS

By dissecting a mouse which has been administered 14C-labelled methotrexate (MTX) and drying the tissues on a gel dryer, whole organs and aliquots of body fluids can be exposed to a phosphor imaging plate. The data obtained was analysed with the software associated with the phosphor imaging system and, by using 14C standards, the amount of 14C per total organ or tissue was quantified relative to other samples. Another widely used method to detect radiolabelled material in vivo is tissue solubilisation (TS) followed by liquid scintillation counting (LSC). This conventional method was compared with DAR.

RESULTS

The new technique described in this communication was found to have a high level of reproducibility (R(2)= 88-95%). Whilst DAR was less sensitive than TS and LSC, trends over time in the biodistribution of 14C-MTX throughout most tissues were consistent between techniques.

DISCUSSION

Whilst TS and LSC was a more sensitive technique, it was labour intensive and expensive in terms of consumables and time when compared with DAR. Dissection autoradiography has the potential to be used to screen quickly large numbers of samples in the biodistribution studies of various conjugates, isomers, derivatives or formulations of a parent compound, following a variety of routes of administration.

Precision of measurement of tissue concentrations by RLG

Existing investigations about the precision of radioluminography (RLG) are restricted to descriptive analysis of the tissue samples. The aim of the present experiments was to obtain a general prospective statement about the precision that the RLG method can achieve. Several pharmaceutical companies in Europe participated in the experiments. Albino rats of various strains were dosed with various (14)C-labeled compounds. Whole-body sections were produced, and blood calibration scales were set up with standard radioactivity sources of dog or rat blood. Photostimulated luminescence was detected using Fuji imaging plate BAS-III. For each organ separately, variability was investigated on each of the levels: rat, section of rat, region within section, and residual, with the help of variance components. The producing company was seen as a fixed factor and adjusted for. A mixed linear model was fitted to the log-transformed data. The variance component (SD estimate) for the residual term gave the desired prospective statement about the achievable precision of the RLG method. Exponential back transformation from the logarithmic to the natural scale transformed the SD estimates to multiplication factors. In total, 29 organs were investigated. The RLG method was comparable in precision to the dissection/combustion method.

Quantitative radioluminography of (125)Iodine whole-body autoradiograms

Radioluminography (RLG) was applied for the quantitation of (125)iodine ((125)I) autoradiograms using the bioimaging analyzer BAS 2000. RLG was performed on 50-microm sections of sagittal whole-body sections of rats and (125)I-radiolabeled erythrocyte calibration scales. Linearity, sensitivity, accuracy, and reproducibility were investigated in comparison to direct (125)I radioactivity measurement. RLG is demonstrated to be a simple, reproducible, and precise analytical tool. However, the spatial resolution of quantitative (125)I RLG is limited by the relatively high average free path of gamma-radiation and X rays.