The Toxicologist - Society of Toxicology

2006, 40:6269-74). Rats received either a low (11.2 mg/kg) or high (27.4 mg/kg)
pyrethroid mixture dose and were sacrificed at 1, 2, 4, 8 or 24 h after dosing. Blood,
liver, fat and brain were removed. Tissues were extracted and analyzed for parent
chemical by LC/MS/MS. For both doses, pyrethroid residues were greater in fat relative
to blood, brain and liver, based on area under the curve from 0 to 24 h (AUC0- 24h ) and maximal concentration (Cmax ). With dose, the AUC0-24h increased 4- to 9fold
in blood, 2- to 3-fold in brain, 2-fold in fat and 4- to 6-fold in liver. There were
similar increases in pyrethroid tissue Cmax with dose. Cypermethrin and cis-permethrin
consistently had the highest AUC0-24h and Cmax in all tissues. trans-
Permethrin consistently had lower AUC0-24h and Cmax in all tissues, which may be
due to increased metabolism relative to the other pyrethroids. Determining the
pharmacokinetics of a mixture of environmental chemicals such as the pyrethroids
is important when assessing the risk to chemicals that have a similar mode of action.
(This abstract does not represent US EPA policy.)
2263 EVALUATING PHARMACOKINETICS AND
BIOAVAILABILITY OF LEVOFLOXACIN IN RATS.
A. Banerjee 1 , K. Kabirov 1 , T. Martín-Jiménez 2 and A. Lyubimov 1 . 1 Toxicology
Research Laboratory, University of Illinois at Chicago, Chicago, IL and 2 College of
Veterinary Medicine, University of Tennessee, Knoxville, TN.
Drug exposure of the antibiotic levofloxacin was examined in Sprague Dawley rats
following oral and intravenous administration. Six female and six male cannulated
rats were randomly assigned to two treatment groups (3 males + 3 females/ group).
One group received a single IV dose of levofloxacin at 25 mg/kg and the other received
a single oral dose at 50 mg/kg. Blood samples were collected, analyzed and
the data used for pharmacokinetic analysis. Intravenous pharmacokinetic profiles
were very similar in males and females, with the exception of systemic clearance,
which was 2222 and 2667 ml/hr/kg in males and females, respectively. Accordingly,
the AUC0-inf values were higher in males (11264 vs. 9464). The estimated values
of C0, Cmax, Vss and terminal elimination half-life were very similar across genders.
The volume of distribution was rather large (Vss: 3.2-3.6 L/kg), which indicates
wide distribution and high affinity of the drug for proteins and perhaps other
molecules in tissue. The absolute oral bioavailability of levofloxacin in rats was 34%
in males and 47% in females which is relevant for a concentration-dependent antibiotic
such as levofloxacin. Accordingly, the AUC0-inf after the oral administration
was lower in males than in females (7668 and 8952 ng*hr/mL, respectively).
The larger gender-related difference in oral pharmacokinetics was in Cmax and average
values in males and females were 1370 and 2733 ng/mL, respectively.
Coefficients of variability for this parameter were rather high (43-59%). The halflife
of the drug after intravenous administration was short (1.5-2.0 hr) but it increased
after oral administration (3.9-4.1 hr). Mean terminal elimination half-lives
were very similar in males and females (3.9 and 4.1, respectively). In conclusion,
bioavailability seen in rats is about 2-fold lower than in humans indicating species
based differences in absorption profile of levofloxacin.
2264 ROLE OF INTESTINAL P450 AND P-GLYCOPROTEIN
IN MODULATING THE BIOAVAILABILITY OF ORAL
LOVASTATIN.
Y. Zhu. Environmental Health Sciences, Wadsworth Center, New York State
Department of Health, and School of Public Health, State University of New York at
Albany, Albany, NY. Sponsor: X. Ding.
Variations in drug metabolism and transport can affect drug bioavailability, and potentially
cause drug toxicity. However, the roles of intestinal enzymes or drug transporters
in controlling the bioavailability of oral drugs are not well defined for many
drugs. Our aim was to determine the specific roles of small intestinal (SI) P450 enzymes
and the P-glycoprotein (P-gp) in the first-pass clearance of lovastatin (LVS),
an anticholesterol drug. In an intestinal epithelium (IE)-specific P450 reductase
(CPR) knockout (IE-Cpr-null) mouse model, which has little P450 activities in the
IE, we found that the rates of LVS metabolism by SI microsomes were reduced by
>90%, compared with wild-type (WT) mice. In vivo clearance of oral LVS was
much slower in IE-Cpr-null than in WT mice; consequently, the bioavailability of
LVS was 14% in IE-Cpr-null mice, but only 5% in WT mice. In contrast, the
clearance rates for intraperitoneally dosed LVS were not different between the two
mice strains. These results demonstrate a predominant role of SI P450 enzymes in
the first-pass clearance of LVS. This conclusion was further supported by additional
data, obtained through pharmacokinetic studies on a liver-specific Cpr knockout
(liver-Cpr-null) mouse model, indicating minimal contribution of hepatic P450
enzymes to the first-pass clearance of oral LVS. The specificity of the impact of the
CPR loss on SI P450 activity was confirmed by the results from pharmacokinetic
studies for pravastatin (PVS), a drug which is similar to LVS, but is not a P450 sub-
486 SOT 2011 ANNUAL MEETING
strate. We then determined whether P-gp contribute to LVS clearance, by using cyclosporin
A (CsA), an inhibitor of both P450 and P-gp, and by taking advantage of
the absence of P450 activities in the SI of the IE-Cpr-null mice. We found that CsA
treatment (oral) significantly enhanced the systemic exposure of LVS in both IE-
Cpr-null and WT mice, a result strongly suggest that P-gp also influences the
bioavailability of oral LVS in mice. (Supported in part by NIH grant GM082978)
2265 PARAQUAT DISPOSITION: A POTENTIAL ROLE FOR P-
GLYCOPROTEIN TRANSPORT.
S. Lacher, F. Cardozo-Pelaez and E. L. Woodahl. University of Montana,
Missoula, MT.
P-glycoprotein (P-gp) is an efflux drug transporter expressed in many tissues important
in xenobiotic disposition. At the blood brain barrier (BBB), P-gp is one of
the mechanisms that protects the brain from toxic substances circulating in the
blood. P-gp has been proposed to play a role in the susceptibility to Parkinson’s disease
(PD), possibly by altering accumulation of neurotoxicants in the brain.
However, evidence is missing as to whether neurotoxicants associated with PD are
P-gp substrates and if P-gp alters their ability to cross the BBB. Our goal is to measure
P-gp transport of neurotoxic xenobiotics associated with PD across the BBB.
Our recent studies focus on the pesticide paraquat with the goal of measuring
paraquat toxicokinetics, paraquat brain accumulation, and markers of neurodegeneration
in FVB mice. We administered paraquat to FVB mice to measure brain accumulation
and estimate paraquat toxicokinetics. Two different groups of FVB
mice were administered a single oral dose of paraquat (10mg/kg and 25mg/kg),
plasma samples were collected at 1, 2, 4, and 8 hours, and the brains were harvested
at 8 hours. Plasma and brain samples were analyzed for total paraquat concentration
using high performance liquid chromatography. We were able to detect
paraquat in the plasma and brain samples in both treatment groups. We found that
the average elimination half-lives of the 10mg/kg and the 25mg/kg treatment
groups were similar at approximately 8 hours, which suggests that there is no dose
effect on half-life. Average brain accumulations were 1.02 +/- 0.21μg and 0.95 +/-
0.44μg after 10mg/kg and 25mg/kg doses, respectively. Our future studies will examine
the toxicokinetics of paraquat disposition in P-gp knock-out (mdr1a-/-
/mdr1b-/-) mice, as well as identifying markers of neurodegeneration. This study
will provide the missing data to measure P-gp-mediated transport of paraquat and
the susceptibility to Parkinson’s disease.
2266 THE IMPORTANCE OF EFFLUX TRANSPORTERS IN
CNS EXPOSURE TO AVERMECTIN INSECTICIDES.
A. M. Dalzell 1 , F. M. Williams 2 , P. Mistry 3 , J. Wright 3 and C. D. Brown 1 . 1 Cell
& Molecular Biosciences, University of Newcastle, Newcastle-Upon-Tyne, United
Kingdom, 2 Medical Toxicology Centre, University of Newcastle, Newcastle-Upon-
Tyne, United Kingdom and 3 Syngenta, Bracknell, United Kingdom. Sponsor: R.
Peffer.
Avermectins are used as agricultural insecticides and as anti-helminthic treatments
in man. Avermectins have good safety profiles in a variety of species; however neurotoxicities
observed in some CF-1 mice initiated investigations into the relevance
of this mouse model for human risk assessment. Susceptible CF-1 mice lack the
functional xenobiotic efflux transporter, P-glycoprotein (mdr1a/P-gp), demonstrating
its importance in neuroprotection. In humans a non-functional MDR1 has yet
to be identified, although the importance of MDR1 in xenobiotic efflux in man
needs to be better understood. In this study we compared two neuroblastoma cell
lines; human SH-SY5Y cells, characterised for uptake and efflux transporter expression,
with mouse N2a cells to identify species differences in avermectin handling.
Intracellular levels of the fluorescent MDR1-substrate H33342 dye in the presence
of avermectins (0-7μM) were compared to the increase (p emamectin > abamectin, in human and mouse
cells. Parallel experiments using a CMFDA dye assay suggest that the avermectins
may be substrates for MRP efflux transporters expressed in both cell types