Active versus passive absorption kinetics as the basis for selective protection of normal tissues by S-2-(3-aminopropylamino)-ethylphosphorothioic acid.

Through an in vivo and in vitro analysis of the absorption kinetics of S-2-(3-aminopropylamino)ethylphosphorothioic acid (WR-2721) in the normal tissues and solid tumors of mice, rats, and rabbits, it has been demonstrated that normal tissues actively concentrate WR-2721 against a concentration gradient, whereas solid tumors passively absorb it, or, if active concentration of WR-2721 is present in tumors, it operates at a far reduced rate relative to normal tissues. These observations can account for the ability of WR-2721 to selectively protect normal tissues against both radiation and alkylating agent injury.