Age response to x-radiation of murine lymphoma cells synchronized in vivo

Hydroxyurea Can Be Used to Increase Mouse c-kit+Thy-1.1loLin−/loSca-1+ Hematopoietic Cell Number and Frequency in Cell Cycle In Vivo

The DNA synthesis inhibitor hydroxyurea (HU) was administered to determine whether it induces changes in the cell-cycle status of primitive hematopoietic stem cells (HSCs)/progenitors. Administration of HU to mice leads to bone marrow accumulation of c-kit+Thy-1.1loLin−/loSca-1+ (KTLS) cells in S/G2/M phases of the cell cycle. HU is a relatively nontoxic, reversible cell-cycle agent that can lead to approximately a threefold expansion of KTLS cells in vivo and approximately an eightfold increase in the number of KTLS cells in S/G2/M. HSCs in HU-treated mice have undiminished multilineage long-term and short-term clonal reconstitution activity.

Hydroxyurea Can Be Used to Increase Mouse c-kit+Thy-1.1loLin−/loSca-1+ Hematopoietic Cell Number and Frequency in Cell Cycle In Vivo

The DNA synthesis inhibitor hydroxyurea (HU) was administered to determine whether it induces changes in the cell-cycle status of primitive hematopoietic stem cells (HSCs)/progenitors. Administration of HU to mice leads to bone marrow accumulation of c-kit+Thy-1.1loLin−/loSca-1+ (KTLS) cells in S/G2/M phases of the cell cycle. HU is a relatively nontoxic, reversible cell-cycle agent that can lead to approximately a threefold expansion of KTLS cells in vivo and approximately an eightfold increase in the number of KTLS cells in S/G2/M. HSCs in HU-treated mice have undiminished multilineage long-term and short-term clonal reconstitution activity.

Biological basis for the interaction of chemotherapeutic agents and radiation therapy

The ultimate goal of combining chemotherapy and radiotherapy is to improve the therapeutic ratio. When chemotherapy is combined with radiotherapy, improved therapeutic effects may occur as a result of the following: (1) spatial cooperation; (2) enhancement of tumor response with less or no enhancement of normal tissue injury, and (3) diminution of normal tissue injury with less or no diminution of tumor response. Possible mechanisms of interaction between chemotherapeutic agents and radiation that result in an enhanced effect include (1) changes in the slope of the dose response curve, (2) decreased accumulation or inhibition of repair of sublethal damage, (3) decreased recovery from potentially lethal damage, (4) perturbation in cell kinetics with an increase of proportion of cells in the sensitive cell cycle phase and proliferative state, (5) decreased tumor bulk and improved blood supply leading to reoxygenation and recruitment and increased radiosensitivity and chemosensitivity, and (6) increased drug delivery and uptake. The combined effects may be influenced by (1) tumor and normal tissue type, (2) drug type, (3) drug dose and schedule, (4) time sequence between drug and radiation administration, (5) radiation dose and fractionation schedule, and (6) radiation dose rate. Most experimental and clinical data suggest that enhanced tumor effects most often result from simple additivity and therefore do not require direct interaction between drug and radiation, whereas enhanced normal tissue effects are observed most often when drugs are administered in close temporal proximity to radiation. Thus, for the optimal therapeutic effect, it would seem more advantageous to administer chemotherapeutic drugs and radiation in a sequential or alternating manner rather than simultaneously.

Radiobiology of the skin: review of some effects on epidermis and hair

Significant doses of ionizing radiation produce an acute skin reaction characterized by erythema, epilation, and dry or moist desquamation with or without erosions. These early acute changes are dose-dependent and reflect damage to the germinative cells of the epidermis and to the cutaneous vasculature. Studies in the pig, for example, have shown that the degenerative phase of cell loss (2-3 weeks) results from reproductive failure in germinative cells and a sharp reduction in the proliferation rate of basal cell "survivors." D0 values for epidermal cells in different species generally range from 100-140 rads. Cell maturation and ascension rates in the suprabasilar layers are largely unaffected. A regenerative phase of cell replacement, characterized by sharply increased cell replication rates, occurs from the 3rd to 5th postradiation weeks. The postregenerative phase of hyperplasia reflects a temporary overshoot of cell density above control levels; a subsequent decrease in hyperplasia indicates feedback control of cellular proliferation. Postradiation changes in the highly proliferative anagen hair matrix cell populations result in hair dysplasia, slowed growth rates, impaired metabolic processes, and alopecia. Dosages of 700-800 rads or more induce some degree of permanent hair loss. G0 telogen matrix cells are 2-3 times more radioresistant than proliferating anagen matrix cells, but may "store" radiation damage for prolonged periods. Altered matrix cell uptake of amino acids, the incidence of dysplasia, and the degree of alopecia occurring after irradiation have all been used as quantitative biological end-points for the general study of cellular radiation effects, as well as studies on the enhancement of or protection against radiation damage provided by certain pharmacologic or physical agents.

The proliferative state of clonogenic cells in the Lewis lung tumour after treatment with cytotoxic agents

The proportion of clonogenic cells from the Lewis lung carcinoma which are in S-phase of the cell cycle has been measured as the fraction killed by a short exposure to hydroxyurea in vitro. Estimates of the proportions of S-phase cells before and 30 min after doses of gamma-radiation of 1000--2000 rad suggest no alternation in the cell cycle age distribution due to these doses of radiation. As the survivors of these high doses of radiation are predominantly hypoxic, the results imply that hypoxic cells have the same cell cycle age distribution as oxygenated cells in Lewis lung tumours. After treatment with cyclophosphamide or CCNU, the proportion of S-phase cells among the survivors exceeds the faction of S-phase cells in untreated populations. This increase is consistent with a relative resistance of S-phase cells to alkylating agents and nitrosoureas.

The induction of transient increases in mitotic rate in murine tissues following prolonged intravenous infusions of hydroxyurea

Intravenous infusions of hydroxyurea were established in mice and maintained for periods up to 48 hr. The influence of different rates of hydroxyurea infusion on the number of viable cells gathered in S phase was studied in eight different mouse tissues. An infusion rate which was sufficiently slow not to block thymidine incorporation completely, resulted in gathering of cells in S phase while offering some protection against hydroxyurea-induced cell death. The duration of the period of DNA synthesis following release from hydroxyurea inhibition appeared to be shortened in some tissues. After the release of hydroxyurea blockades maintained for 12-24 hr, each of the tissues showed sharp increases in mitotic activity and peak mitotic index values were as much as twenty times greater than values found in tissues of control animals. An important finding was that the time of maximal mitotic activity for different tissues after release of blockade could differ by many hours.