24 
to 1 microgram per kilogram may be performed at P3 + E2CL or P2 + EK2; 
cloning of DMAs coding for toxins with an LD 50 of 1 microgram to 100 
micrograms per kilogram nay be performed at Pi + EXl. PI + EKl containment 
nay be used for specified enterotoxins. ENAs coding for proteins with an 
LD 50 of greater than 100 microgretns per kilogram nay be handled as nontoxins 
un^r the Guidelines. 
Dr. Gill said the effects of intraintestinal production of cytotoxic toxins 
might include ( 1 ) direct danage to the intestinid lining and ( 2 ) passage 
through the lining into the bloodstream. In the absence of information 
on the effect of E. ooli elaborating toxin in the human intestine, the 
working group basi? t)ie classification on available data from humans, 
primates, and small aninals, generated by intravenous or parenteral e^lminis- 
tration of toxins. A listing of toxins by potency has been prepared and 
is available from ORDA. In addition, the working group outlined a procedure 
for evaduating toxins to be added to the list in the future. The procedure 
is edso available from ORDA. 
Dr. Gill outlined the types of data used to determine toxicity: 
( 1 ) Hiiw> toxicity, if known, %cxdd be paramount in fixing containment 
levels. 
(2) If hun^n toxicity is not known, it would be inferred from assays of 
toxicity to other primates. 
(3) If neither humem nor prinate toxicity is knovn, it would be inferred 
from the LD50 of the most sensitive of three small animal species 
(mice, guinea pigs and rabbits). 
Dr. Gill said that in those cases in which there is hurran data, nan is 
not significantly more sensitive to the toxin than the most sensitive of 
three small animal species (mouse, rabbit or guinea pig). 
Ikr. Gill pointed out to RAC that diphtheria toxin appears to have an LD 50 
of 100 nanograms or less per kilogram body weight in hurans. The working 
group designated P3 ♦ EXl containment, but Dr. Gill asked RAC to consider 
trihether this containment was appropriate. 
Dr. Levine reiterated that although the proposed classification is based 
on pharmaoologiced potency, the toxin delivery system is highly important 
to the pathogenicity of a toxin producing orgcmism in nature. 
Dr. Nightingede as)ced if the toxins prohibited by the prc^x^ed classifica- 
tion (botulinun, tetanus. Shigella neurotoxin) would be the only prohibited 
toxins. Dr. Gill replied that these toxins are the only ones currently 
known to have an LD 50 of 100 nanograms or less per kilogram (other than 
diphtheria which is right on the borderline). If other toxins are found 
in the future to have an LD50 in this range, they would be put on the list 
euxJ the cloning of their gene vould be prohibited. 
( 113 ) 
