STUBBORN TUMOR YIELDS TO BACTERIAL CELL KILLER
BioWorld via NewsEdge Corporation : Hungarian dermatologist Moritz Kaposi (1837-1902) would hardly recognize the cancer that bears his name. When he reported in 1872 what became known as Kaposi's sarcoma (KS), it was a rare, slow-growing malignancy of elderly Jewish and Italian men, marked by purplish spots on their toes and legs. (See
BioWorld Today, Aug. 6, 1998, p. 1.)
For the past two decades, KS has been known as an aggressive, life-threatening disease, endemic in equatorial Africa - particularly Uganda and Zaire - epidemic among HIV-infected men with late-stage AIDS. In the U.S. and Europe, nine out
of ten cases of KS occur in homosexual or heterosexual males with AIDS, 30 percent of whom come down with the disease.
Its treatment is something of a Catch-22. Interferon works well in 50 percent of patients, but only if they are immunocompetent - which, by definition, few AIDS victims are. By the time they receive a diagnosis of KS, their HIV infection may have progressed to a point where death ultimately intervenes from other causes.
Actually, KS is often the first inkling that an HIV-positive person has advanced to full-blown AIDS. A barely raised round or oval purplish, pinkish or reddish nodule on the upper body, mouth or gut, soles and genitalia, is the giveaway hallmark. Extensive bleeding from internal organs often follows.
Current anti-HIV therapies have somewhat diminished the incidence of KS, but it remains a serious public health problem, and a distressing affliction to individual patients. In recent years, Over 30 clinical trials, from Phase I to Phase III, have tested a wide range of therapeutic approaches; none achieved total elimination of tumors.
Target: AIDS-Associated Kaposi's Sarcoma
Now, a preclinical report in Nature Medicine for July 1999 describes " complete eradication" of KS in tumor-bearing mice treated with a novel form of therapy. Its title is, "Interleukin-4 receptor-directed cytotoxin therapy of AIDS- associated Kaposi's sarcoma tumors in xenograft model." The article's senior author, Raj
Puri, performed these experiments in CEBRA - the Center for Biologic Evaluation and Research of the FDA, in Bethesda, Md., in close collaboration with the National Cancer Institute's Laboratory of Molecular Biology.
Molecular biologist and clinical oncologist Robert Kreitner, at the NCI, is a co- author of the paper. "We designed and produced this IL-4 toxin," he told BioWorld Today. "Raj Puri has been working with IL-4 receptors, and testing murine IL-4 toxins, for over a decade. For the last few years, he has been doing animal models in his lab, looking at the anti-tumor activity in vivo.
They and others had noticed that KS cells invariably expressed large amounts of the interleukin-4 receptor (IL-4 R), but normal cells do not. The immune system's T lymphocytes and mast cells express the IL-4 cytokine, which damps down the growth of KS cells in vitro, but in the clinic, IL-4 showed only subclinical activity.
So, to pump up this bird-dog attraction by the IL-4 cytokine for KS tumors, they tried the strategy of hooking a potent bacterial toxin onto that molecule, so it would kill KS cells on contact. This potent poison came from Pseudomonas exotoxin, which, Kreitner observed, "helps Pseudomonas aeruginosa kill patients in hospitals with septic shock."
The aggressively tumorigenic KS cell line came from human surgical specimens.
By manipulating the PE protein's sequence's and fusing it to a " re-jiggered" IL- 4 sequence, they constructed a highly cytotoxic, high-affinity, recombinant anti-KS package. KS cells expressed some 1,800 KS-avid IL-4R molecules per tumor cell.
Then it was preclinical in vivo testing time.
The team injected their human KS cell line into nude mice, which - lacking immune defenses - were unable to reject them. They inoculated the animals with the xenografts followed by their putative, KS-clobbering IL-4/PE toxin package at one of three sites: the peritoneum, the bloodstream and the tumor itself. A fourth cohort of infected animals got no therapy, and served as controls.
The six intraperitoneal mice got these xenograft and immunotoxin injections on alternate days, for a total of three escalating therapeutic doses. By day 17, the co- authors reported, "all three doses (50, 100 and 200 micrograms per kilogram) induced complete response of tumors." Meanwhile, the tumors of control animals "continued to grow exponentially." The highest-dose complete responders remained tumor-free for at least 114 days, the trial's duration.
Result: Highest Dose, Right Into Tumor
A similar test by the intravenous route of administration was less dramatic. Low- medium and medium-dose therapy showed partial tumor regression, but then the cancers reappeared, albeit smaller. The 200 mg/kg [[Greek letter mu]] top dose proved lethally toxic. It indicated, the paper observed, that "the intraperitoneal route is better and safer than the intravenous route."
There then remained local injection of the recombinant immunotoxin directly into the tumor mass itself. In a six-day regimen, this eradicated the AIDS- associated KS completely. In the highest-dose cohort, all six animals showed no tumor regrowth on day 82, the last day of the experiment, "with no apparent toxicity."
Besides their wildly growing tumors, control mice experienced severe weight loss - a hallmark of AIDS-associated KS. This cachexia increased with an increase in tumor size, whereas their littermates who received therapy put on
substantial body weight.
Because human IL-4 does not bind murine IL-4 receptors, but does bind primate
cells, the FDA co-authors tried their immunotoxin construct on monkeys. They administered 50 or 200 mg/kg of their IL-4 toxin intravenously to monkeys every other day for three doses. "They tolerated the therapy well," the paper reported.
Observing that "this is the first report to our knowledge in which in vivo AIDS- KS tumors were completely regressed by IL-4R directed therapy," the FDA researchers concluded, "we are considering initiating a Phase I clinical trial of IL-4 toxin for the treatment of aggressive AIDS-KS."
But Kreitner's IL-4/PE construct is already being tested on patients with a different disease. "Raj had done a lot of the animal models for other cancers," he recalled, "including glioblastoma multiforme, a brain tumor model, before the
KS model. And both of these are similar in that the tumor is injected intratumorally. n
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