Radioprotective Effects of S-Phosphorylated Thiols

Modification of neutron-induced hematopoietic effects by chemical radioprotectors

Nine drugs, WR-347, WR-1065, WR-2529, WR-2721, WR-3689, WR-44923, WR-151327, WR-109342, and WR-168643, were assayed for intraperitoneal pharmacological toxicity, hematopoietic lethality, and spleen stem cell survival in C57/Bl mice in conjunction with whole-body fission neutron irradiation. In addition, WR-3689, WR-109342, and WR-168643 were used with per os administration to determine hematopoietic lethality. Drug toxicities after intraperitoneal administration, as determined by LD50's, ranged from 37 mg/Kg for WR-109342 to 2901 mg/Kg for WR-2529. Following oral administration drug toxicities ranged from 58 mg/Kg (WR-109342) to 1816 mg/Kg (WR-3689). Dose modification factors as determined by LD50(30)'s following intraperitoneal administration ranged from 1.08 for WR-347 to 1.49 for WR-2529. Of the drugs orally administered, WR-168643 was the best protector with a DMF of 1.51. Spleen stem cell survival, which was performed only after intraperitoneal drug administration, provided no significant differences in dose modification factors.

Sodium hydrogen-S-(3-amino-2-hydroxypropyl) phosphorothioate (WR-77913): toxicity and bone marrow radioprotection

Experiments have been carried out to compare the toxicity and radioprotective effect of sodium hydrogen-S-(3-amino-2-hydroxypropyl) phosphorothioate (WR-77913) with those of S-2-(3-aminopropylamino)ethyl phosphorothioic acid (WR-2721). The drugs were given intraperitoneally to 12 week-old female BALB/c mice 30 minutes before whole body irradiation. Lethality at 30 days was the endpoint used. The drug LD50/30 was 678 mg/kg for WR-2721 and 3574 mg/kg for WR-77913. The LD50/30 for WR-77913 combined with 500 mg/kg of WR-2721 was 3328 mg/kg. The LD50/30 for misonidazole was 380 mg/kg when given in combination with 500 mg/kg of WR-2721 and 801 mg/kg when combined with 2200 mg/kg of WR-77913. Protection of bone marrow by WR-77913 and WR-2721 was comparable at doses close to the maximum tolerable dose (MTD, drug dose lethal to 10% of the animals at 30 days), but WR-77913 gave better protection at 35% of the MTD. These characteristics of low toxicity, non-additive toxicity with WR-2721, less toxicity in combination with misonidazole and adequate bone marrow protection at 25% of the MTD, make WR-77913 a protector worthy of further investigation.

Toxicity and radiation protective effect of WR-77913 in BALB/c mice

The toxicity and radiation protective effect obtained by intraperitoneal injection of WR-77913 have been investigated in BALB/c mice. The toxic LD50/30 was 3574 mg/kg. When WR-77913 was given 30 min before whole body irradiation adequate protection was observed against bone marrow and gut death. Dose modifying factors of 1.91 and 1.76 for bone marrow and 1.95 and 1.80 for gut death were obtained with drug doses roughly equivalent to one half and one quarter of the maximum tolerable doses (MTD). No protective effect was observed against central nervous system injury. Preliminary experiments with the EMT6 tumor show limited tumor protection with a dose of WR-77913 equivalent to 70 per cent of the MTD. Because of its low toxicity, adequate bone marrow and gut protection at doses equivalent to one quarter MTD and limited protection of the EMT6 tumor, WR-77913 deserves further investigation to determine its value in multiple drug dose regimens and its capability to protect other normal tissues.

The application in radiation therapy of substances which modify cellular radiation response

Compounds to be considered as potential adjuvants in radiotherapy should indicate a strong mechanistic rationale for a differential response between tumor and normal tissues. Drugs which selectively radiosensitize hypoxic cells might be exploited in radiotherapy to increase the sterilization of resistant hypoxic areas of solid tumors which have outgrown their vascular supply. Several compounds have been shown to selectively radiosensitize hypoxic mammalian cells in vitro but their application in clinical studies in presently limited by their toxicity at the high drug dosages required to effect the sensitization. Phase I and Phase II clinical studies have now been completed with the sensitizer, metronidazole. This drug was tolerated by patients at dosages of 6 g/m2 three times a week for 3 weeks with a minimum of side effects, and survival of patients with glioblastoma multiforme treated with fractionated radiotherapy was significantly increased when the sensitizer was used in combination with the radiation. Drugs which selectively radioprotect oxygenated cells might also be exploited in therapy be permitting the use of higher radiation doses and thereby incurring more damage to the resistant hypoxic cells in tumours. Again, drug toxicity appears to be a limiting factor with this approach. If toxicities are not additive, combinations of radiosensitizers and radioprotectors might prove more effective than either individual approach.