With 38 more calendar days to go before its 60-day cut-off date, the National Cancer Institute (NCI) already hasreceived "several" inquiries from companies respondingto an offer that appeared in the Aug. 10, 1995 FederalRegister.
That announcement invited "a pharmaceutical orbiotechnology company to develop novel therapeutics forAIDS and other human and animal diseases of retroviraletiology, based upon a newly identified, highly conservedHIV target protein." It offers the chosen candidate aCooperative Research and Development Agreement(CRADA) partnership.
NCI Biochemist Louis Henderson nutshells the problemhis invention seeks to solve: "The most attractive partabout it," he told BioWorld Today, "is that HIV is ahighly mutagenic virus, and its ability to mutate has verymuch frustrated efforts to develop drugs and vaccinesagainst AIDS. By rapidly mutating a target protein, HIVcan evade a drug."
He and his co-workers at the NCI's AIDS VaccineProgram in Frederick, Md., have zeroed in on a particularstructural feature of a nucleocapsid protein that residesdeep in HIV's interior, which, Henderson said, "simplycannot mutate."
He added, "This particular target feature is one of themost highly conserved elements in all retrovirology. It'sfound in all lentiviruses and oncornaviruses, including allof the immunodeficiency viruses _ human, simian,feline, bovine and so on.
"That's the major reason why we selected thisnucleocapsid protein as a target, and went to work findingcompounds that could hit it, and so frustrate HIV'spropensity to develop drug-resistant mutants.
"The reagents that we're talking about," he continued,"induce a covalent modification in the nucleocapsidprotein, and so render the virus non-infectious. In thatway, these drugs or drug candidates, of course, are quitedifferent from the usual class of anti-AIDS therapeutics_ AZT, protease inhibitors and the like."
Such currently used compounds, Henderson pointed out,"are reversible inhibitors. They will diffuse into the virus,bind to the target, and then sometimes diffuse out. Theyare ineffective against AIDS due to the emergence ofdrug-resistant viral mutants."
He observed: "Our new drugs, once they react with atarget, induce a chemical modification _ and that's aforever thing."
Fingering The Zinc-Finger
How do they do it? "These compounds attack the zinc-finger structures in retroviral nucleocapsid proteins,"Henderson said, explaining:
"HIV's 52-amino-acid nucleocapsid peptide chainincludes two stretches of 14 amino acids each. Evenlyspaced along each of these sequences are four aminoacids, three cysteines and one histidine, that loop aroundand bind to a single zinc ion. That loopy conformationlooks finger-shaped, hence, zinc fingers.
"HIV-1," he pointed out, "has two zinc fingers, bothnecessary for infectivity. A hit in either one inactivatesthe protein."
After cloning and expressing nucleocapsid proteins, theNCI team discovered that "the zinc-finger metal-ioncoordination complex is capable of donating electrons tomany classes of electron receptor. "Among these,"Henderson recalled, "we found compounds that areknown to be quite non-toxic. We've tested some of thesepotential drug candidates orally in whole-animal modelsystems, virus-infected mice and monkeys, and foundthem to be effective in reducing viremia load, andattacking the target protein."
Because that protein's chemistry, he pointed out, "is thesame, regardless of which retrovirus you start with, youcan just as well test the compounds' effects using non-human pathogens. We used equine infectious virus andMoloney murine leukemia virus, rather than live HIV,which can be a little dangerous."
By the same token, "the new drugs would be effectiveagainst other human retroviral infections, besides HIV.
The compounds they've tested against the target protein'szinc finger structure deliver a one-two body blow to theHIV virion's life cycle. "All viruses," Hendersonobserved, "have to solve a fundamental problem, which isto assemble their proteins so they can recognize oneanother. Then, they must also assemble their nucleic acidout of the vast milieu of nucleic acids that exist in the cellthey've infected."
He continued: "If the retroviral zinc finger is destroyed,disrupted or reacted, by mutagenesis or by thesecompounds, the virus may assemble but it doesn't haveits own nucleic acid."
In nature, that target protein can never mutate, but theNCI team alters it by site mutagenesis. "Every mutantvirus we make that affects these metal-ion chelationresidues is absolutely dead," Henderson said.
He continued, "One zinc-finger-attacking compound thatworks, though not particularly well, is already an FDA-approved drug. And you can administer it orally."
From CRADA-Enhanced Drugs To Human Trials
He hastened to point out that the compound in questionwas approved for a totally different purpose, applied byother researchers entirely. "It's not yet in clinical use,"Henderson observed, "but it should be approved as ananti-retroviral agent. We're trying to get that approvalnow, but it's not completed yet."
Nor is it ideal. The NCI group is looking to industry fordevelopment of more efficacious reagents. They havepurchased five dozen off-the-shelf oxidizing agents "thatcan work, but are in no way specific for the virus. Whatwe want to do now," Henderson said "is get morespecific. We would like to reduce whatever toxicity theremight be, reduce the extent to which these drugs mightattack other targets, and enhance the probability that theywill react with the virus.
"To do all that," he said, "what we need is apharmaceutical or biotech company that is willing tosynthesize analogs, then start testing to see if we aremaking improvements."
From "selected variants that we think have a goodprobability of being useful, we'd try to carry on intoanimal, and ultimately human, trials. We would want theCRADA company to help us with that as well, financiallyor otherwise, depending on their capabilities."
Note: Interested parties may address inquiries concerningthis CRADA opportunity to NCI technology developmentspecialist Cindy Fuchs at (301) 846-5465. n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.