© 1991 Oxford University Press
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Glyphosate Skin Binding, Absorption, Residual Tissue Distribution, and Skin Decontamination

*Department of Dermatology, University of California San Francisco, California 94143-0989
California Primate Research Center Davis, California
Received June 15, 1990; accepted December 28, 1990
Glyphosate Skin Binding, Absorption, Residual Tissue Distribution, and Skin Decontamination. Wester, R. C., Melendres, J., Sarason, R., McMaster, J., and Maibach, H. I. (1991). Fundam. Appl. Toxicol. 16, 725732. Glyphosate is a broad-spectrum postemergence translocated herbicide. Its interactions with skin and potential systemic availability through percutaneous absorption was studied by skin binding, skin absorption, residual tissue distribution, and skin decontamination. Glyphosate in a final formulation (Roundup) undiluted and diluted with water 1:20 and 1:32, would not partition into powdered human stratum corneum (<1%). In vitro percutaneous absorption through human skin into human plasma as receptor fluid was no more than 2% over a concentration range of 0.5154 µg/cm2 and a topical volume range of 0.0140.14 ml/cm2. Disposition of glyphosate following iv administration of 93 and 9 µg doses to rhesus monkeys was mainly through urine excretion, 95 ± 8 and 99 ± 4% in 7 days, respectively. Percutaneous absorption in vivo in rhesus monkey was 0.8 ± 0.6% for the low dose (25 µg/cm2) and 2.2 ± 0.8% for the high dose (270 µg/cm2). No residual 14C was found in organs of the monkeys euthanized 7 days after the topical application. Washing the skin application site with soap and water removed 90 ± 4% of applied dose, and washing with water only removed 84 ± 3% of applied dose. Both soap and water and water only were equal in ability to remove glyphosate from skin over a 24 hr skin application period. About 50% of the initially applied dose could be recovered after 24 hr. Glyphosate is very soluble in water and insoluble in most organics (octanol/water log P = 1.70) and therefore not compatible with the lipid-laden stratum corneum. This is consistent with the low skin binding and skin absorption and also consistent with the efficient removal from skin with soap and water or water-only wash.