Comparative distribution of marrow CFU-e and CFU-gm progenitors in different anatomic sites in the dog

Alternative testing systems for evaluating noncarcinogenic, hematologic toxicity

Hematopoietic tissues are the targets of numerous xenobiotics. Clinical hematotoxicity is either a decrease or an increase in peripheral blood cell counts in one or more cell lineages--a cytopenia or a cytosis, respectively--that carries a risk of an adverse clinical event. The purpose of in vitro hematotoxicology is the prediction of these adverse hematologic effects from the effects of the toxicants on human hematopoietic targets under controlled experimental conditions in the laboratory. Building on its important foundations in experimental hematology and the wealth of hematotoxicology data found in experimental oncology, this field of alternative toxicology has developed rapidly during the past decade. Although the colony-forming unit-granulocyte/monocyte neutrophil progenitor is most frequently evaluated, other defined progenitors and stem cells as well as cell types found in the marrow stroma can be evaluated in vitro. End points have been proposed for predicting toxicant exposure levels at the maximum tolerated dose and the no observable adverse effect level for the neutrophil lineage, and several clinical prediction models for neutropenia have developed to the point that they are ready for prospective evaluation and validation in both preclinical species and humans. Known predictive end points are the key to successful comparisons across species or across chemical structures when in vitro dose-response curves are nonparallel. Analytical chemistry support is critical for accurate interpretation of in vitro data and for relating the in vitro pharmacodynamics to the in vivo pharmacokinetics. In contrast to acute neutropenia, anemia and acute thrombocytopenia, as well as adverse effects from chronic toxicant exposure, are much more difficult to predict from in vitro data. Pharmacologic principles critical for clinical predictions from in vitro data very likely will apply to toxicities to other proliferative tissues, such as mucositis.

Leukocyte response to toxic injury

Exposure to a variety of drugs and toxins can induce hematopoietic damage. These agents exert their effects through several distinct mechanisms including destruction or suppression of hematopoietic stem cells, cytotoxic destruction of rapidly proliferating precursor cells, immune-mediated hematotoxicity, altered hematopoietic microenvironment, genetic mutation, and microvascular injury. Some toxins consistently produce suppression of granulopoiesis in a dose-dependent manner, whereas others produce idiosyncratic reactions. Although all types of injury tend to result in granulocytopenia, the time course of the changes varies with the type of injury. With acute destruction of the proliferative pool of granulocytes, neutropenia develops within 7 days and recovery occurs within days after discontinuing treatment. With stem cell injury, the onset of hematotoxicity is variable and damage is often permanent. Immune-mediated reactions may occur acutely or after months or years of treatment with a drug. Drugs or toxins that act as mutagens can induce a variety of hematopoietic disorders including aplastic anemia, myelodysplasia, and leukemia. Therefore, the time course of onset of leukopenia after drug or chemical exposure and the rapidity of hematopoietic recovery give clues to the mechanism by which granulopoiesis is suppressed.

Development and application of in vitro models of hematopoiesis to drug development

In vitro models of hematopoiesis are used increasingly in investigative hematopathology. Such models complement in vivo animal testing and have been shown to be predictive for hematotoxicity associated with anticancer and antiviral agents in humans. In vitro models of hematopoiesis consist of short-term cloning assays for various hematopoietic progenitor cells and long-term functional assays for the marrow hematopoietic microenvironment. In our laboratories, the cloning assays have been used as investigative tools to study the pathogenetic mechanisms of drug-induced blood disorders and as screening systems to investigate the possible hematotoxic potential of candidate drugs in various animal species. Data in support of these applications are presented in this paper.

Canine BFU-e progenitors: adaptation of a reproducible assay and anatomical distribution

In vitro cloning assays are used increasingly in investigative hematotoxicology and in screening candidate compounds for their hematotoxic potential. To expand these applications, a practical cloning assay for erythroid burst-forming units (BFU-e) that uses a microplasma clot (MPC) system was adapted to the dog, a species used extensively in experimental hematology and drug development. This system offers the advantage over the methylcellulose and soft agar culture systems of allowing specimen fixation and, therefore, morphological and cytochemical evaluation. The distribution of BFU-e among various anatomic sites was assessed using the MPC cloning system, which was modified to optimize the BFU-e growth. BFU-e growth required only erythropoietin (Epo) in the culture medium and there was no need for an exogenous source of burst-promoting activity (BPA). The cloning efficiency was linearly proportional to the plating concentrations of Epo and marrow mononuclear cells (MMC) over a range of 0 to 3 U Epo and 1 x 10(5) to 3 x 10(5) MMC per ml of culture, respectively. Increases in concentrations of Epo and MMC beyond these levels were not associated with linear growth. The addition of transferrin and spleen-conditioned medium containing a mixture of growth factors (including BPA) reduced BFU-e growth. The relative concentration of BFU-e was comparable among samples collected from the iliac crest, femur, and humerus. Serial cultures performed on individual dogs were highly reproducible and there was little variation in BFU-e activity among dogs of comparable age. It was concluded that the MPC system is a practical and reproducible cloning system for early (BFU-e), as well as late erythroid colony-forming units (CFU-e) in the dog. The concentration of BFU-e appears comparable throughout the active marrow; therefore, various anatomic sites can be used interchangeably for serial quantitative analysis of this progenitor.