Protection of mouse jejunal crypt cells by WR-2721 after small doses of radiation

The ability of WR-2721 to protect jejunal crypt cells after single doses and multifractionated doses of radiation was studied. Effective dose survival curves for jejunal crypt cells were constructed over the dose range of 230 to 1600 cGy. WR-2721 was given 30 minutes before each fraction, in a regimen consisting of 200 mg/kg before the first radiation fraction, followed at 3 hr intervals by 100 mg/kg for a total of 12 drug doses for the largest number of fractions. Fractionation protocols were designed with common dose fractions in regimens with different fraction numbers, allowing a test of the hypothesis of equal effect per fraction and an estimate of the initial number of clonogens per crypt in both the drug treated and non-drug treated mice. The hypothesis of equal effect per fraction could not be rejected in either the drug or non-drug treated mice. An average number of 137 clonogens per crypt was estimated for the non-drug treated mice and 81 clonogens per crypt in the drug treated mice; the difference between these two values was not significant. The protection factor decreased with decreasing dose ranging from a high of 1.47 (95% C.L. = 1.44 to 1.50) after a single dose of 2000 cGy to a low of 1.21 (95% C.L. = 1.08 to 1.37) after 200 cGy. Analysis of the data using either the linear quadratic (LQ) or two-component (TC) model of cell survival showed that WR2721 was not dose-modifying over the dose range tested. Analysis using the LQ model showed that both beta and alpha were modified by WR-2721, by 50% and 20% respectively. These data indicate that protection by WR-2721 can be expected to decrease with dose although there is some protection after clinically relevant doses.