Human trials are planned next year for high-densitylipoprotein (HDL) as a treatment for septic shock. Following invivo testing in mice, the cholesterol-clearing protein begantoxicity studies in larger mammals last month to provide safetydata to the FDA for permission to conduct Phase I clinicals.
HDL is popularly known as the "good-guy" lipoprotein thatremoves cholesterol and wards off atherosclerosis. A glass ofwine a day reputedly helps keep the cardiologist away byboosting HDL levels. Conversely, low-density lipoprotein (LDL)promotes heart disease.
A paper in this week's Proceedings of the National Academy ofSciences (PNAS) proposes a new therapeutic role for HDL:ridding the blood of sepsis patients from life-threateningbacterial endotoxins. It describes how researchers at CornellMedical Center's Rogosin Institute injected HDL into mice,which then survived endotoxin injections that killed controlanimals.
"Rogosin is a not-for-profit research and medical treatmentfacility that specializes in heart and kidney disease, employingextracorporeal therapy," the PNAS paper's first author,biochemist Daniel Levine, explained to BioWorld.
Such ex vivo blood-cleansing suggested infusing exogenous HDLinto the circulation to Levine and co-author Thomas Parker. "Itwas common back in 1988," Parker told BioWorld, "to speculatethat by pheresing a patient, removing 50 percent of his LDLand then sending him home with an injection of HDL, we'd pullout the bad, put in the good, and hopefully accelerate reversetransport of cholesterol from the atherotic lesions."
Cholesterol has a lot in common chemically with the death-dealing lipopolysaccharide (LPS) toxins released by Gram-negative bacteria. Both are lipids to the core, presenting similartargets for HDL expulsion from the blood into the liver, thenout the door.
In sepsis, with bacteria pumping toxins into the bloodstream,the body's immune system fights back, too often withinflammatory overkill. Cytokines rush out to take on theantigenic enemy and rain down friendly fire on their own side,resulting in septic shock and, frequently, death.
Topping the roster of these cytokines are tumor necrosisfactor-a and interleukins 1 and 6. Reports in the literature toldLevine and Parker that HDL can bind 10 to 1,000 times moreLPS than is found in the blood of sepsis patients.
So they reasoned that if HDL can mop up the LPS before sepsisbegins, it could blunt the cytokine attack and save the patient,where monoclonal antibodies to bacterial endotoxin have so farfailed.
Mice proved their point. "It turns out to be a lot easier to studysepsis than atherosclerosis," observed Levine.
The Rogosin in vivo experiments challenged two breeds of micewith HDL injections. One was a transgenic strain carrying agene encoding HDL's precursor molecule, human apolipoprotein.Naturally, these mice expressed elevated HDL levels in theirblood. The second group of animals received intravenousinfusions of synthetic or "reconstituted" HDL or peptide analogs,free of the trace endotoxins that contaminate the protein inevery healthy bloodstream.
"These reconstituted HDLs provided immediate protectionagainst a lethal dose of Salmonella endotoxin that killed 80percent of controls," said Levine.
Survival rates among the 101 transgenic mice varied directlywith the HDL levels they expressed, with 50 to 60 percent ofthe 15 highest expressors defeating the LPS challenge. Doublingthe plasma level of HDL trebled or quadrupled the rate ofsurvival.
TNF-a released into the circulation closely PP and inversely PPtracked survival rates, showing that HDL can curb the cytokinecascade provoked by endotoxin.
Since the research reported in PNAS, Levine, Parker and theirco-workers have just reported in abstract form that theirsynthetic HDL is active against all varieties of Gram-negativebacteria, and that mice with naturally low HDL levels are morevulnerable to endotoxin.
Levine suggested that "this may be relevant to the clinicalsepsis situation, where people who are sick come into thehospital with low HDLs to begin with."
Safety tests in larger animals have just begun. Last monthRogosin's drug-development licensee, Glaxo Group ResearchLtd. of Ware, England, began preliminary regulatory safetystudies of exogenous HDL as an endotoxin scavenger in rats anddogs.
"This is not one continuous study, but a number of studies, eachof very short duration, with the design determined by whathappened in the previous one," Neville Spurling, who headsGlaxo's toxicology department in England, told BioWorld. "Wewill go on until we have a feel for how we should conduct fullregulatory studies."
Whether these Phase I trials will take place in England or theU.S. has yet to be decided. Levine, who directs Rogosin's clinicallaboratory, said he expects them to begin before the end of1994.
Spurling said that "Glaxo's interest at the moment is purelyseptic shock, but it is not shutting the door on other potentialapplications."
Neither is Levine. "We see HDL as the transporter of morethings than just cholesterol," he said. "We've been thinking interms of a lipid waste-disposal system. Endotoxin is just thefirst example of that we're studying."
Glaxo and Rogosin signed a worldwide exclusivity agreement inMarch, granting the British company a license to manufactureand market Rogosin's patented, reconstituted HDL peptide.Glaxo will defray development of the synthetic high-densitylipoprotein and fund further research in it at Rogosin for threeyears.
Levine is listed as first inventor on U.S. Patent No. 5,128,318,dated July 7, 1992. It specifies that the reconstituted HDLparticle "is useful [inter alia] in removing excess lipid solublematerial from a subject to which it is administered."
One lipidologist, who asked to remain anonymous, finds it "anexciting clinical prospect."
The researcher described several caveats, however:
-- Manufacturing HDL in sufficient quantity will be problematiceither for extracting apolipoprotein from blood or making arecombinant version.
-- Gram-negative endotoxin is not the only cause of septicshock; will the HDL also deal with Gram-positive toxins?
-- In critical sepsis situations, can HDL be administered inquantity high enough and soon enough to reverse the threat?Can it be effective prophylactically?
-- Might dosage adequate to remove the toxin be itself toxic toorgans such as the liver, which clears lipoproteins from thebody?
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.