With reproductive biologists cloning sperm-specific antigensfrom mice, rabbits, primates and humans, the concept ofcontraception by immunization is moving closer to reality. Thelatest progress report, "Molecular cloning of the human andmonkey sperm surface protein PH-20," appeared recently inthe Proceedings of the National Academy of Science (PNAS). Itdescribes a membrane protein positioned at the tip of guineapig spermatozoa that enables a sperm to stick to the zonapellucida, the outer sheath of its target ovum.
As the result of a research group's work at the University ofConnecticut Health Center, in 1988, PH-20 became the firstpregnancy-blocking antigen to achieve 100 percent successarresting fertility in a mammalian model. In the group'sexperiment, 25 female and six male guinea pigs received twoinjections of GP-20 a month apart, Paul Primakoff, the team'sleader and principal author of the PNAS paper, told BioWorld.
A month later, the team mated the 25 females to unvaccinatedmales. Not one of them produced a litter, while almost all 36control females did. The immunity reversed in females after sixto 18 months and in males after about a year.
Since that initial experiment, the Connecticut group hasimmunized more males and allowed them to mate with twofemales each. Again, the experimental females did not getpregnant, but those mated to the control males did, saidPrimakoff.
"A lot of people for years now have been looking for a proteinthat will work," the paper's co-author, Diana Myles, toldBioWorld. "The only problem is, no one but our lab has reallyfound one that's achieved high contraceptive efficacy."
Now, as PNAS reported, that efficacy is being extended fromguinea pigs to cynomolgus monkeys and humans. With 510 and509 amino acids, respectively, monkey PH-20 differs fromhuman by 53 residues PP a high (90 percent) sequence identity."Though monkeys are evolutionarily close to humans, humansperm do not bind to the zona pellucida of eggs from monkeys,"Primakoff pointed out.
A sperm protein proposed as a contraceptive antigen must be100 percent specific to the testis so it won't elicit autoimmuneattack. Primakoff determined this to be true of PH-20 in guineapigs, and recently in monkeys and humans as well. "Thisfinding suggests the feasibility of developing human PH-20 asan antigen for a human contraceptive vaccine," he concluded.
But primates come first. The PNAS paper reported the cDNAsequence encoding the protein for use in the primate, Mylessaid. "Our goal right now is to make a true, high-fidelity copy ofthe monkey protein, which we can then produce in quantity fortrials."
Besides refining the form of the simian antigen, the researchershave to choose the optimum expression host. "E. coli lacks thetrue fidelity of a eukaryotic protein," Myles observed. "Insectcells, baculovirus, don't process it exactly correctly. So we'retrying to do it in a mammalian system."
Primakoff foresees starting cynamolgus monkey trials in amatter of months. "At some point, after success in the primates,obviously we'll try it in humans," he added. "But there will be alot of things to work out first."
At least one other reproductive biologist is edging close tohuman trials of a contraceptive vaccine. Erwin Goldberg ofNorthwestern University has cloned and chemicallysynthesized a testis-specific enzyme, lactate dehydrogenase(LDH), which is presumably involved in sperm metabolism,motility and possibly egg penetration.
Three months ago, he and his group completed a two-year testof this pregnancy-blocking LDH, combined with a carriermolecule, in 14 female baboons plus 15 controls. "We had about75 to 80 percent reduction in fertility in the immunizedbaboons," Goldberg told BioWorld, "and this was reversible withtime."
He has since combined the synthetic LDH peptide with asynthetic promiscuous T cell epitope. "Now our formulation iscompletely made by chemical synthesis, from simple aminoacids in the lab," he said. "This permits quality control. Onedoes not have to isolate the antigen from natural sources or byrecombinant technology."
Meanwhile, to define the function of the LDH enzyme in spermactivity, Goldberg is making a construct that will knock out itsgene in mice, "and (we will) then see what happens to theirspermatogenesis, if they don't have the enzyme."
The Northwestern group has several issued patents and theConnecticut group has one pending. Florence Haseltine, directorof the Center for Population Research at the National Instituteof Child Health and Human Development, told BioWorld thatPrimakoff has a collaborative research agreement with OrthoPharmaceutical Corp.
Pharmaceutical companies have expressed only moderateinterest in his emerging contraceptive technology, Goldbergsaid. "In my opinion, they are at the present somewhatcautious about vaccines, not only for contraception, but evenagainst diseases, because of potential liability issues," he toldBioWorld.
He deplores this situation because of the worldwide need foradditional contraceptive technologies. "The population problemis probably the most serious problem facing the world over theforeseeable future," Goldberg added. And overpopulation is notjust a problem in the developing world. "In terms ofreproductive policy, the U.S. is a Third World Country,"Haseltine said. "Forty percent of all pregnancies occur inwomen under age 20, which matches the rate of anydeveloping country."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.