Thomas W. Menard

Radioprotection of normal tissues against gamma rays and cyclotron neutrons with WR-2721: LD50 studies and 35S-WR-2721 biodistribution.

The ability of WR-2721 to protect mice against two modes of death following whole-body radiation with 137Cs gamma rays or d(22)+Be neutrons was examined. For single fractions, 400 mg/kg WR-2721 was administered prior to irradiation. In two-fraction exposures, the dose was 275 mg/kg given prior to each fraction. Dose modification factors (DMFs) were calculated as ratios of LD50 values. For single fractions of gamma rays, the DMF was 1.74 for the LD50/7 end point and for LD50/30, the DMF for single fractions was 2.25. For two fractions 3 hr apart, it was 1.88. For single fractions of cyclotron neutrons, the DMF was 1.32 for LD50/7. Measured with the LD50/30 end point, the DMF for single neutron doses was 1.41 and for two fractions, 1.19. Substantial radioprotection of bone marrow and intestinal epithelium against cyclotron neutrons was seen in these investigations. Biodistribution studies were done following ip injection of 35S-labeled WR-2721 into C3H mice bearing RIF-1 tumors. Blood levels peaked at 10 min after injection and declined thereafter. Most normal tissues achieved maximum levels of 35S at 30 to 60 min postinjection and high concentrations were retained in most tissues for up to 2 hr. Assuming that all 35S is in parent compound or dephosphorylated radioprotective metabolites, the concentration of protector (milligram per gram tissue) in various organs at 30 min postinjection ranked as follows: kidney greater than submandibular gland much greater than liver = lung greater than gut greater than heart much greater than blood greater than skin greater than tumor greater than brain. High levels of 35S were achieved and retention times were long in certain normal tissues which respond at early or late times postradiation and may be dose limiting in radiotherapy: kidney, liver, salivary gland, and lung. These combined observations suggest that there is potential for protecting dose-limiting, late-responding normal tissue in the radiotherapy of human cancer with both neutrons and conventional radiotherapy.

Biodistribution of the radioprotective drug 35S-labeled 3-amino-2-hydroxypropyl phosphorothioate (WR77913).

3-Amino-2-hydroxypropyl phosphorothioate (WR77913), a less toxic phosphorothioate radioprotector than WR2721, has been labeled with 35S. The biodistribution of a radioprotective dose of 800 mg/kg was determined in C3H mice bearing RIF-1 tumors as a function of time after intraperitoneal injection and was expressed as percentage injected dose/gram (% ID/g). Levels of 35S in the blood peaked 10 min after injection, and radioactivity in most tissues was highest at 15 min. Label in most tissues declined markedly between 15 and 60 min, but in gut, salivary glands, tumor, and brain, the levels of radioactivity remained quite stable over 1 hr. At 30 min after injection the highest levels of labeled drug were found in submandibular salivary glands, gut, and kidney, with the lowest level in brain. Tumors had approximately the same amount of label as blood, muscle, skin, and esophagus. Two principal differences between the distribution of label from WR77913 and WR2721 were defined. Although blood levels of 35S-WR2721 also peaked 10 min after injection, the 10-min blood levels achieved for WR77913 were more than fourfold greater than those attained by WR2721. Maximum levels of WR2721 occurred in most tissues 30 to 60 min after administration of the drug, compared to 15 min for WR77913. The basis for these differences remains to be determined, but these results suggest that the optimum interval between administration of WR77913 and irradiation may be shorter than for WR2721.

Comparative biodistribution and radioprotection studies with three radioprotective drugs in mouse tumors

The organ level biodistribution and tumor radioprotective properties of three drugs have been compared: WR-2721 (NSC 296961), WR-3689 (NSC 327729), and WR-77913 (NSC 318809). The three drugs have similar distribution patterns in normal mouse tissues. At 30 minutes after intraperitoneal injection, highest levels of 35S from radiolabeled protector are found in kidney and submandibular salivary gland, with lowest levels in brain and moderately low values in tumor and skin. Three of four tumors examined take up less WR-3689 than the other two protectors. For the three protectors, the dose modifying factors for the RIF-1 tumor irradiated in vivo and assayed in vitro are 1.5-1.7, but do not vary as predicted by differential uptake of drug into this neoplasm. In RIF-1, WR-3689 is taken up most avidly, but the three drugs tend to be equally protective.

Synthesis, biodistribution, and autoradiography of radiolabeled S-2-(3-methylaminopropylamino)ethylphosphorothioic acid (WR-3689)

35S- and 3H-labeled S-2-(3-methylaminopropylamino)ethylphosphorothioic acid (WR-3689) have been synthesized in our laboratory and used to study organ and cellular level distribution in C3H/Km mice bearing RIF-1 tumors. Tissue biodistributions obtained with 35S-WR-3689 showed that blood levels peak at 15 min postinjection and decline gradually over 60 min. At 30 min after drug injection the highest uptake is in kidney and submandibular salivary gland, with lowest levels in brain and moderate to low levels in the RIF-1 tumor, comparable to levels in skin and muscle. High resolution diffusible substance autoradiography with 3H-WR-3689 reveals a homogenous distribution of label over cells in liver and lung and nonuniform distribution of silver grains over the cytoplasm of cells in the kidney cortex, parotid and submandibular salivary glands, and small intestine. There are no indications of preferential nuclear location of label from protective drug in any tissue. Correlations of biodistribution and autoradiography data with measures of radioprotection in different tissues will be useful in interpreting mechanisms of radioprotection with this phosphorothioate.

Autoradiolysis of iodinated monoclonal antibody preparations

We investigated the effects of ionizing radiation on the immunointegrity of antibody fragments (Fab) because large amounts of high specific activity 131I may damage the proteins. We found that 1000 Gy of external 137Cs gamma radiation was sufficient to destroy 80-90% of the immunointegrity of the initial preparation. This effect was also produced by internally added [131I]NaI in a quantity sufficient to provide the same radiation absorbed dose. Since radioiodinated monoclonal antibodies labeled to high specific activity are being evaluated for radioimmunotherapy, the above observation is significant since high levels of internal radiation occur with therapeutic doses of 131I-labeled antibody. Human serum albumin in low concentration (2%) added to the iodinated antibody solutions was successful in preventing loss of immunoreactivity and can be used to protect and stabilize therapeutic quantities of radiolabeled monoclonal antibody preparations.

Radioprotection against cataract formation by WR-77913 in gamma-irradiated rats

Protection by WR-77913 against radiation-induced cataract formation in rats was observed following intraperitoneal (i.p.) administration of drug (1160 mg/kg) 15-30 min before exposure to 15.3 Gy of Cs-137 whole head irradiation. Control groups included irradiated, non-protected animals, and sham-irradiated aging controls. Protection was documented photographically and by analysis of eye lens constituents. All non-protected irradiated animals developed dense cataracts throughout the lens between 90-120 days post-irradiation, while WR-77913 protected animals developed minimal lens opacification through 200 days post-irradiation. No opacification in aging controls was seen. Lens protein analysis by Lowry assay and size exclusion HPLC showed radioprotected and aging control animals were similar in protein content, distribution of total and soluble protein, and degree of lens hydration. This contrasted significantly with cataractous lenses of non-protected animals. In cataractous lenses, the soluble protein concentration in the 25-43 K dalton range was approximately 10% of that found in radioprotected or aging control lenses. Hydration was substantially higher in cataractous lens. These results indicate that WR-77913 protects against lens opacification, protein insolubilization, and hydration in lenses of irradiated animals. Biodistribution studies with [S-35]-WR-77913 showed ocular uptake of drug within 15 minutes after i.p. injection, which remained relatively constant through 60 min. The relative order of drug concentration for individual eye components was: globe greater than total eye approximately equal to humor greater than lens. Although the mechanism of radioprotection observed remains to be elucidated, WR-77913 clearly prevents radiation-induced cataracts in rats. The potentially significant clinical use for this radioprotective compound is being investigated further.

Rat parotid gland pathophysiology following 137Cs irradiation.

Changes in rat parotid gland function were measured between 3 and 30 days following exposure to 1800 R of137 Cs irradiation to the head. Glandular fluid secretion capability as indicated by volume of secretion, maximum rate of secretion, and duration of secretion following pilocarpine stimulation decreased concomitantly with gland weight following irradiation. Thus gland weight is probably indicative of residual glandular potential for fluid secretion following irradiation. The relation between salivary sodium concentration and flow rate was assumed to be indicative of glandular electrolyte transport capability. Salivary sodium concentrations were not elevated over control values at any flow rate. Therefore, there is no evidence for an irradiation-induced defect in the ductal sodium resorption mechanism. Rather, the observed salivary sodium changes are consistent with a decrease in the relative glandular proportion of acini to ducts.

Analysis of S-35 labeled WR-2721 and its metabolites in biological fluids☆

Studies with WR-2721 and related compounds have been hindered by the lack of a suitable assay for the drug and its major metabolites. We have developed a chromatographic method which requires no derivatization for the separation and detection of WR-2721, the free thiol, its symmetrical disulfide and other mixed disulfides. Our procedure involves ion-pairing for separation of ionizable compounds by causing polar molecules to become more lipophilic and hence separable using reverse phase HPLC. Detection is based upon liquid scintillation counting of S-35 incorporated during the synthesis of the parent compound. This method requires no pre-column preparation of samples and, by detecting the S-35 label, eliminates the chance that a coeluting species could interfere with detection, as might occur with post-column derivatization. Chromatography was done using a 10 micron C8RP column and 35% MeOH/65% 0.0113M NaH2PO4, 0.005 M hexanesulfonate, pH 5.9, flowing at 1 ml/min. Half-minute fractions were collected into scintillation vials for counting. Retention volumes for the various compounds were: column breakthrough (3.5 ml), WR-2721 (4.5 ml), WR-1065 (9 ml), and WR-33278 (24 ml). This analytical technique employing radiotracers can be used to study radioprotective mechanisms by time dependent measurements of the tissue distribution and chemical form of labeled drug. Such chemical information can then be correlated with biological measures of radiation protection.