Absorption - Pesticide Biotransformation and Disposition - page 39

vehicle appears to be slightly less or not significantly different from absorption from
soil. However, for several other pesticides (e.g., DDT, organic arsenicals), soil appears to
reduce percutaneous absorption of the pesticide. The interactions between soil and sev-
eral of these pesticides are subsequently described in more detail, but note that in vitro
skin models are, in general, not very predictive of in vivo absorption when exploring
these interactions ( Wester and Maibach, 1998 ).
Although DDT is no longer widely used in the United States, residues in soil
are still detectable and human contact with contaminated soil can result in DDT
exposure. One study demonstrated that in vivo absorption of DDT in rhesus monkeys
was significantly less from soil (3.3% dose) than from acetone vehicle (18.9%) ( Wester
et al., 1990 ). The absorption of DDT in acetone in rhesus monkey is not significantly
different from DDT absorption in humans (10.4% dose; Feldmann and Maibach,
1974 ). In vivo absorption from acetone or soil was not similar to in vitro absorption
(0.1%). However, in vitro experiments demonstrated that 18.1% penetrated skin with
acetone and 1.0% penetrated skin with soil. Less than 1.0% dose partitioned into the
receptor phase, demonstrating that the skin barrier in addition to the soil is rate limit-
ing and that in vitro skin models may not be useful for predicting DDT absorption
in vivo.
Unfortunately, only in vitro dermal absorption studies are available for organic
arsenicals. One study demonstrated that as much as 12.4% dose of monosodium
methyl arsenate (MSMA) and disodium methyl arsenate (DSMA) penetrated mouse
skin within 24 h from aqueous vehicles over a wide dosage range ( Rahman and
Hughes, 1994 ). Of this amount, only 4% was absorbed into the receptor fluid. In the
presence of soil (690 ppm), penetration through mouse skin was reduced to not more
than 0.48 and 0.22% for MSMA and DSMA, respectively. Increasing MSMA and
DSMA levels in soil from 690 to 6900 ppm increased skin content, but decreased the
percentage of applied dose in skin. Whereas absorption into receptor fluid was very
low for MSMA (0.01%), it was not detectable for DSMA. Topical application of aque-
ous solutions (20, 100, and 250 l) of 10 g of dimethylarsenic acid (DMA) to mouse
skin resulted in 5.16-25.22% dose in receptor fluid and 1.95-15.67% dose in skin tis-
sue within 24 h ( Hughes et al., 1995 ). However, when DMA (690 ppm) was applied
with soil, absorption was reduced to 0.08% in the receptor fluid and 0.45% in skin.
The influence of soil was, however, not observed with inorganic arsenic. In vivo
percutaneous absorption of arsenic as H 3 A 5 O 4 in water in rhesus monkeys (2.0-6.4%)
was somewhat comparable to in vitro absorption (1.9%) in human skin ( Wester et al.,
1993a ). However, the soil vehicle did not influence absorption in rhesus monkeys (3.2-
4.5%) or human skin in vitro (0.8%), although absorption in these skin models is not
comparable. The relative similarities in the partition coefficient of arsenic from water
to stratum corneum and from water to soil probably explain why absorption from
water was similar to absorption from soil.