Absorption - Pesticide Biotransformation and Disposition - page 35

insect repellent while working with pesticides. The active ingredients in many com-
mercial sunscreens contain UV absorbers (e.g., titanium dioxide and zinc oxide),
which could act as penetration enhancers of 2,4-D, paraquat, parathion, malathion
( Brand et al., 2002, 2003, 2007 ). Other studies have demonstrated that the active
ingredient in many insect repellents, DEET, enhances transdermal delivery of drugs
and toxicants ( Moody et al., 1987; Windheuser et al., 1982 ). Some studies have dem-
onstrated that DEET can act as a transdermal accelerant of 2,4-D-amine ( Moody
et al., 1992 ). Recent studies in our laboratory have, however, determined that DEET
blocked permethrin absorption and inhibited carbaryl absorption in acetone, but not
in dimethyl sulfoxide mixtures ( Baynes and Riviere, 1998; Baynes et al., 1997 ). The
insecticide synergist piperonyl butoxide, which is often formulated with some insec-
ticidal products, was also shown to enhance carbaryl absorption ( Baynes and Riviere,
1998 ). These diffusion studies further demonstrated that piperonyl butoxide does not
enhance absorption through inert latex membranes, but does so in porcine skin sec-
tions. This observation suggests that some chemical-biological interaction or other
mechanisms (e.g., irritation) may occur in skin to enhance the absorption of pesticides.
An expected, but important finding in these carbaryl experiments was that increased
dilution of the carbaryl formulation with water, especially in the presence of the sur-
factant sodium lauryl sulfate (SLS), enhanced carbaryl absorption. The penetration-
enhancing effect of SLS was also observed with parathion ( Qiao et al., 1996 ).
In addition to the formulation additives, agrochemicals may contain isomers, homo-
logs, or breakdown products that form after synthesis and/or formulation and during
storage ( Chambers and Dorough, 1994 ). Although these impurities can potentially alter
the toxicity and toxicokinetics of the pesticide, many toxicology and dermal absorption
studies have ignored these impurities and used the pure rather than the technical-grade
pesticide. There is evidence that technical-grade malathion can be more lethal (eight-
fold difference) in rats than the purified form ( Umetsu et al., 1977 ). Other studies have
demonstrated that organophosphates such as malathion and fenitrothion can potentiate
the toxicity of the carbamate insecticide carbaryl ( Takahashi et al., 1987 ).
Previous metabolism studies in isolated perfused porcine skin flaps ( Carver et al.,
1990 ) demonstrated a significant first-pass metabolism of parathion to p -nitrophe-
nol and paraoxon, and that these metabolites may be present simultaneously during
absorption of parathion. Environmental exposure to parathion is never to pure para-
thion because spontaneous degradation occurs during storage. When mixtures of
parathion and its metabolites were dosed and then assayed for parathion and its two
metabolites across pig skin in vitro, significant interactions were detected. In general,
the nontoxic metabolites p -nitrophenol and 1-naphthol can significantly enhance
the absorption of the parent compounds, parathion and carbaryl, respectively ( Baynes
and Riviere, 1998; Chang et al., 1994 ). Surprisingly, p -nitrophenol did not enhance
the absorption of paraoxon; this toxic metabolite of parathion appears to decrease the