Pectin helps fight cancer's spread.

Home canners rely on pectins, plant-derived gelling agents, to set their jams and jellies. One day physicians may also rely on pectins -- to jam certain receptors on the surface of cancer cells.

Blocking these receptors appears to prevent malignant cells circulating with the blood from seeding tissues wit new tumors, called metastases, according to a study published this week. Because fewer people die from a primary tumor than from the growths they spawn, preventing metastasis remains one of oncology's leading targets.

For reasons scientists do not understand, most solid tumors seventually shed cells into the blood. Though the body kills most of these cells, any that escape the bloodstream can seed new cancers far from the initial tumor.

Certain cell-surface proteins, called lectins, help tumor cells clump. The larger the clump, the more likely it will lodge in some tissue as a metastasis, notes Avraham Raz of the Michigan Cancer Foundation (MCF) in Detroit.

Lectins that bind galactoside (a sugar-based molecule) play a role in the cell clumping of many cancers. Raz speculated that because pectin has numerous galactoside side chains, it may bind cancer cells (via their lectins) into clumps. By contrast, pectins possessing a single lectin-binding site might inhibit clumping. Together with David Platt, also at MCF, Raz created such pectin molecules by cleaving the normally branch-shaped citrus pectin into linear subunit with a single, free galactoside. In the March 18 JOURNAL OF THE NATIONAL CANCER INSTITUTE, the pair reports that in incubated melanoma cells, citrus pectin indeed increases cell clumping, whereas the modified pectin does not.

The most dramatic evidence of pectin's influence, however, appears in the lungs of mice autopsied 17 days after they had received injections of melanoma cells -- with and without some form of pectin. Four times as many metastases peppered the lung of mice receiving injections containing normal pectin as those injected with the tumor cells only. Animals receiving the modified pectin developed the fewest and smallest metastases (see photo).

"This is the first report ever describing the use of [nontoxic] plant products to try to prevent metastasis," Raz told SERVICE NEWS.

Indeed, altering the implantation of circulating cancer cells with modified pectin "is a very clever idea," says Lance A. Liotta of the National Cancer Institute in Bethesda, MD. This promising approach also represents one of the few designed to stop metastases by means other than killing tumor cells, adds Hynda K. Kleinman of the National Institute of dental Research, in Bethesda.

Kleinman notes, however, that the Michigan team's cell-culture experiments fall short of demonstrating how the modified pectin works in mice. It's a lesson she learned last year, when Japanese researchers published data demonstrating that a nontoxic, anti-metastasis drug her team was working on (SN: 4/15/89, P.228) operates through a mechanism totally different from the one suggested by several cell-culture tests.