BREAST CANCER RESISTANCE PROTEIN - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 332

inhibit ABCG2 competitively. 119 A list of reported ABCG2 inhibitors is provided in

Table 12.1.
Several studies have noted the impact of amino acid 482 on inhibitor potency.
While confirming the effects of reported ABCG2 inhibitors in stably-transfected
HEK-293 cells, we noted that novobiocin was effective in reversing ABCG2-mediated
transport of fluorescent substrates only in cells transfected with wild-type but not
mutant ABCG2. 44 The effects of amino acid changes at position 482 were observed
for biricodar, developed as a modulator of both Pgp and MRP1, and the taxane
derivatives ortataxel and tRA96023. 120 − 122 Amino acid 482 mutations have also been
shown to alter the ability of some flavonoids to inhibit ABCG2. 115
The growing number of substrates and inhibitors for ABCG2 is not surprising,
considering previous experience with Pgp and its known promiscuous nature. The
ability to transport a wide range of unrelated compounds supports its suspected role
in protecting cells from xenobiotics, and suggests that like Pgp, drugs bind ABCG2
in a large central cavity rather than in a lock-and-key conformation. The parallel be-
tween ABCG2 and Pgp in drug transporter paradigms allowed the rapid identification
of inhibitors but probably has hindered the actual clinical development of ABCG2
inhibitors.

12.7. TISSUE LOCALIZATION OF ABCG2

After its initial discovery in resistant cancer cell lines, investigators pursued vary-
ing lines of inquiry to determine the location, expression, and physiological role of
ABCG2 in vivo. In the initial report identifying ABCG2, Doyle et al. performed
Northern blot analysis in commercially prepared human samples representing 16 dif-
ferent tissues. 9 Using cDNA probes to examine expression of ABCG2 mRNA, they
found the highest expression in placental tissue. Lower levels were observed in tissue
from the brain, prostate, small intestine, testis, ovary, colon, and liver. Transcripts
were undetectable in tissues from the heart, lung, skeletal muscle, kidney, pancreas,
spleen, thymus, and peripheral blood leukocytes. We later confirmed these results,
noting high expression in the central nervous system, liver, adrenal gland, placenta,
prostate, testes, and uterus. 123 Lower levels were detected by Northern blot in the
small and large intestine, stomach, lung, kidney, and pancreas. 123
Maliepaard et al. examined ABCG2 expression in normal tissue and cancer
cell lines by immunohistochemistry using the BXP-21 and BXP-34 monoclonal
antibodies. 124 Again, high expression was found in placental tissue and specifically
in the placental syncytiotrophoblast. Strong positive staining was also observed in
the colon, and positive staining was seen in the small intestine, biliary canaliculi (but
not hepatocytes or bile ductules), breast tissue, venous endothelium, and in capil-
laries. Comparison was also made of mRNA expression by semiquantitative reverse
transcriptase polymerase chain reaction (RT-PCR) with the IHC assay. RT-PCR re-
sults generally correlated with IHC staining with BXP-21 and BXP-34 antibodies,
except that mRNA levels were higher in tissues with greater blood vessel density (e.g.,
ovary, cervix, small intestine) and were lower in tissues with a smaller endothelium