Introduction:Drug metabolism varies with a diurnal rhythmicity. Intestinal drug transporters located on the small bowel enterocyte apical and basal membranes are very important in drug pharmacokinetics, controlling influx and efflux of many drugs. Extrusion of drugs into the intestinal lumen contributes significantly to clearance of many drugs, including chemotherapeutic agents.Aim: We hypothesized that drug transporters, in common with other enterocyte nutrient transporters, vary with a diurnal rhythmicity. Timing of drug and chemotherapy administration with these rhythms would allow better bioavailability, with delayed clearing of the drug.Methods:50 Male Sprague-Dawley rats were acquired and acclimatized to a 12-hour light dark cycle. Jejunal mucosa was harvested by killing animals at 3-hour intervals (n=6-7) over 24 hours, starting at Zeitgeiber time ZT0 (lights-on, 7am). mRNA was extracted, reverse transcribed and subjected to qPCR. The following transporters were studied: multidrug resistance protein MDR1, 3; multidrug resistance-associated protein MRP1-3; organic anion transporter 2B1 (OATP-B); organic cation/carnitine transporter 2 (OCTN2); Breast cancer resistance protein (BCRP); monocarboxylate transporter 1 (MCT1); and peptide transporter 1 (PepT1). qPCR products were expressed relative to Actin.
Results: All transporters were expressed in the small intestine, across a very wide range of expression levels (approximately 4 log difference). Profound diurnal rhythms were observed for MCT1, MDR1, OCTN2, PepT1, MRP2, and BCRP signal (see Table). All peaked during fasting. The remaining transporters showed no clear diurnal rhythmicity, although notably these transporters were expressed at a much lower levels.
Conclusions: We describe for the first time diurnal rhythmicity in transcriptional expression of major drug transporters, which helps establish diurnal rhythms as a novel in vivo model for studying physiological drug transporter regulation. Their diurnal expression rhythm suggests that timing of drug administration is likely to impact drug bioavailability, including chemotherapy agents.

TRANSPORTER	GENE NAME	SUBSTRATE EXAMPLES	RELATIVE EXPRESSION	PEAK TIME	FOLD CHANGE	p-VALUE
MCT1	SLC16A1	Monocarboxylates, aspirin	++	ZT0	5.1	0.0009
MDR1	ABCB1	Small hydrophobic drugs, digoxin	+++	ZT6	4.4	0.00006
OCTN2	SLC22A5	Verapamil	+++	ZT0	4.3	0.00002
PepT1	SLC15A1	Small peptides, beta-lactams	+++	ZT6	4.3	0.0001
MRP2	ABCC2	Ceftriaxone, doxorubicin	+++	ZT6-12	2.4	0.0006
BCRP	ABCG2	Purine analogs, methotrexate	++++	ZT0	1.9	0.02
OATP-B	SLCO2B1	Organic anions, pravastatin	+++	None	N/A	N/A
MRP1	ABCC1	Hydrophobic peptides, vincristine	+	None	N/A	N/A
MRP3	ABCC3	Glucuronides, vincristine	++	None	N/A	N/A
MDR3	ABCB4	Phospholipids, vinblastin	+	None	N/A	N/A