Comparing the Hematopoetic Syndrome Time Course in the NHP Animal Model to Radiation Accident Cases From the Database Search.

Since controlled clinical studies on drug administration for the acute radiation syndrome are lacking, clinical data of human radiation accident victims as well as experimental animal models are the main sources of information. This leads to the question of how to compare and link clinical observations collected after human radiation accidents with experimental observations in non-human primate (NHP) models. Using the example of granulocyte counts in the peripheral blood following radiation exposure, approaches for adaptation between NHP and patient databases on data comparison and transformation are introduced. As a substitute for studying the effects of administration of granulocyte-colony stimulating factor (G-CSF) in human clinical trials, the method of mathematical modeling is suggested using the example of G-CSF administration to NHP after total body irradiation.

Linking the human response to unplanned radiation and treatment to the nonhuman primate response to controlled radiation and treatment.

A key difficulty in developing countermeasures against radiation-induced health impairments is the clear lack of controlled clinical studies, due to the relatively low number of radiation victims worldwide. Instead, established and accepted animal models, as well as the recommendations of national and international expert panels and committees, are the main sources of information. Therefore, the development of countermeasures requires comparison of data from many sources and accumulation of information consistent with the U. S. Food and Drug Administration's "Animal Rule." A new approach is the comparative analysis of human data from the SEARCH (System for Evaluation and Archiving of Radiation Accidents based on Case Histories) database and data from nonhuman primate (NHP) animal model studies. The SEARCH database contains 824 clinical cases from 81 radiation accidents in 19 countries. This exceptional collection of clinical data from accidentally radiation-exposed persons is analyzed regarding clinical signs and symptoms of radiation-induced health impairments. To analyze the time course of radiation syndromes, clinical parameters common to the SEARCH and NHP databases have to be assigned into comparable categories of clinical severity for each species. The goal is to establish a method for comparison of human and NHP data, validate the NHP data as a surrogate for human efficacy/clinical studies, and open a way for the extraction of diagnostic and treatment methods for humans after radiation exposure according to relevant regulations.

Hemopoietic response to low dose-rates of ionizing radiation shows stem cell tolerance and adaptation.

Chronic exposure of mammals to low dose-rates of ionizing radiation affects proliferating cell systems as a function of both dose-rate and the total dose accumulated. The lower the dose-rate the higher needs to be the total dose for a deterministic effect, i.e., tissue reaction to appear. Stem cells provide for proliferating, maturing and functional cells. Stem cells usually are particularly radiosensitive and damage to them may propagate to cause failure of functional cells. The paper revisits 1) medical histories with emphasis on the hemopoietic system of the victims of ten accidental chronic radiation exposures, 2) published hematological findings of long-term chronically gamma-irradiated rodents, and 3) such findings in dogs chronically exposed in large life-span studies. The data are consistent with the hypothesis that hemopoietic stem and early progenitor cells have the capacity to tolerate and adapt to being repetitively hit by energy deposition events. The data are compatible with the "injured stem cell hypothesis", stating that radiation-injured stem cells, depending on dose-rate, may continue to deliver clones of functional cells that maintain homeostasis of hemopoiesis throughout life. Further studies perhaps on separated hemopoietic stem cells may unravel the molecular-biology mechanisms causing radiation tolerance and adaptation.

Computer-assisted severity of effect assessment of hematopoietic cell renewal after radiation exposure based on mathematical models.

After accidental radiation exposure, one of the most significant health impairments is the partial or complete failure of the blood forming systems. Depending on the degree of damage, a suitable therapy must be prepared in time. This requires the assessment of the degree of damage of the blood-forming system and, in particular, of the stem-cell pool. A new approach for assessing the degree of hematopoietic impairment based on dynamic reactions of blood counts immediately following radiation exposure is presented. Cell kinetic mathematical models of blood cell turnover, neural networks, and expert-assessed clinical data records of historical radiation accidents are combined to provide a method for automatic classification of patients and to assign them to clinically related categories of severity. Using this computer-assisted approach, it is possible to distinguish those patients that are likely to restore their blood-cell formation autochthonously from those that need stem-cell transplantation.

Stem cells, multiorgan failure in radiation emergency medical preparedness: a U.S./European Consultation Workshop.

The concern of the public regarding terrorist actions involving nuclear emergencies resulted in the reopening of the discussion regarding the best ways to cope with the inevitable health impairments. Medical experts from the US and from Europe considered it of importance to harmonize at an international level the diagnostic and therapeutic approaches regarding the radiation-induced health impairments. The present contribution is the result of the first U.S./European Consultation Workshop addressing approaches to radiation emergency preparedness and assistance, which was held recently at Ulm University, Ulm, Germany. Discussions dealt with the assessment of the extent of damage after total body exposure and, in particular, the quantity and quality of the damage to the hematopoietic stem cell pool. Secondly, the pathogenesis of the multiorgan failure was considered because of the organ-to-organ interactions. Thirdly, approaches were considered to harmonize the "triage-methods" used on an international level using the "Response Category" approach as developed for the European Communities. These discussions lead to the conclusion that there is a strong need for continuing education of physicians, nurses, and support personnel to address the issues posed by the management of patients suffering from radiation syndromes. Finally, the discussions expressed the need for more international cooperation in research and development of more refined methods to treat patients with any type of radiation syndromes.

Hematopoietic cell renewal systems: mechanisms of coping and failing after chronic exposure to ionizing radiation.

On the occasion of the first international workshop on systems radiation biology we review the role of cell renewal systems in maintaining the integrity of the mammalian organism after irradiation. First, 11 radiation emergencies characterized by chronic or protracted exposure of the human beings to ionizing irradiation were "revisited". The data provide evidence to suggest that at a daily exposure of about 10-100 mSv, humans are capable of coping with the excess cell loss for weeks or even many months without hematopoietic organ failure. Below 10 mSv/day, the organisms show some cellular or subcellular indicators of response. At dose rates above 100 mSv/day, a progressive shortening of the life span of the irradiated organism is observed. To elucidate the mechanisms relevant to tolerance or failure, the Megakaryocyte-thrombocyte cell renewal system was investigated. A biomathematical model of this system was developed to simulate the development of thrombocyte concentration as a function of time after onset of chronic radiation exposure. The hematological data were taken from experimental chronic irradiation studies with dogs at the Argonne National Laboratory, USA. The results of thrombocyte response patterns are compatible with the notion of an "excess cell loss" (compared to the steady-state) in all proliferative cell compartments, including the stem cell pool. The "excess cell loss" is a function of the daily irradiation dose rate. Once the stem cell pool is approaching an exhaustion level, a "turbulence region" is reached. Then it takes a very little additional stress for the system to fail. We conclude that in mammalian radiation biology (including radiation medicine), it is important to understand the physiology and pathophysiology of cell renewal systems in order to allow predicting the development of radiation induced lesions.