10 
Dr. Murphy said these data suggest Eh coll host- vector systems carrying the 
hybrid gene are not harmful to guinea pigs. Table 2 data suggest that up to 
1000 equivalent diphtheria toxin doses of conjugate protein challenges to 
guinea pigs cause no effect. He said one animal died during the course of 
these experiments; autopsy showed no evidence of the pathology of the adrenal 
glands characteristic of diphtheria toxin intoxication. 
Dr. Gill said the crucial question in review is whether lack of human toxicity 
can be predicted frcm the data submitted by Dr. Murphy. He sard one question 
is whether the a-MSF-diphtheria toxin hybrid protein could be lethal to humans 
or whether an exposed individual might become albino. He said it is not kncwn 
whether destroying cells possessing cr-MSH receptors would be lethal to a human. 
Dr. Habig asked whether human ce-MSH would be recognized by guinea pig a-MSH 
receptors. Dr. Murphy said human a-MSH is recognized by guinea pig melanocytes. 
Dr. Gottesman asked Dr. Murphy what cells having a-MSH receptors would have been 
encountered by the toxin during the guinea pig tests. Dr. Murphy said skin 
melanocytes in the G2 phase of the division cycle are sensitive to diphtheria 
toxin. Cells in the brain also have MSH receptors but would not be exposed m 
these tests because of the blood-brain barrier. 
Dr. Kopecko asked whether the guinea pig has a-MSH receptor-possessing cells 
in the peritoneum. Dr. Mirphy said guinea pigs are very sensitive to mtra- 
pentoneally introduced diphtheria toxin. 
Dr. Gill asked Dr. Murphy if he knew whether guinea pig melanocytes were 
destroyed during the test. Dr. Murphy said he did not know if melanocytes had 
been destroyed . 
Dr. Levine asked how long the animals had been observed during these tests. 
Dr. Murphy said the animals had been observed for at least 150 hours. This is 
approximately 24 hours beyond the observation period enployed when animals are 
challenged with one minimal lethal dose equivalent of diphtheria toxin. 
Dr. Murphy said diphtheria toxin is only fetal to sensitive cells in the G2 
portion of the cycle since sensitive cells only express a-MSH receptor in the 
G2 phase of division. Normal melanocytes have a very lew basal rate of protein 
synthesis and replicate about once a year. Therefore, only a very small fraction 
of the melanocyte population would be expressing the a-MSH receptor at a given 
time. In order to envisage fatality, either a long colonization period by 
toxin producing bacteria or a long serum half life of the a-MSH-diphthena 
toxin hybrid molecule would have to be hypothesized . 
Dr. Habig asked whether the ar-MSH-diphtheria toxin hybrid protein could enter 
brain cells through hydrophobic insertion if administered mtracerebrally 
or mtraneural ly. Dr. Murphy said the conjugate toxin probably is as potent 
as the CDM 45 fragment when injected mtracerebrally or mtraneural ly. 
Dr. Habig said diphtheria toxoids with an intact A chain possess a feir amount 
of residual enzymatic activity. He asked Dr. Murphy if he had any data on the 
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