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Vol. 63, Issue 4, 870-877, April 2003
Pharmacia Research and Development, St. Louis, Missouri
Two compounds (celecoxib and valdecoxib) from the diarylheterocycle
class of cyclooxygenase inhibitors were radiolabeled and used to
characterize their binding to cyclooxygenase-1 (COX-1), cyclooxygenase-2 (COX-2), several single-point variants of COX-2 (Val523Ile, Tyr355Ala, Arg120Ala, Arg120Gln, Arg120Asn) and one triple-point variant of COX-2 [Val523Ile, Arg513His, Val434Ile (IHI)]. We demonstrate highly specific and saturable binding of these inhibitors to COX-2. Under the same assay conditions, little or
no specific binding to COX-1 could be detected. The affinity of
[3H]celecoxib for COX-2
(KD = 2.3 nM) was similar to the
affinity of [3H]valdecoxib
(KD = 3.2 nM). The binding to COX-2
seems to be both rapid and slowly reversible with association rates of
5.8 × 106/M/min and 4.5 × 106/M/min
and dissociation rates of 14 × 10
3/min
(t1/2 = 50 min) and 7.0 × 10
3/min (t1/2 = 98 min)
for [3H]celecoxib and [3H]valdecoxib,
respectively. These association rates increased (4- to 11-fold) when
the charged arginine residue located at the entrance to the main
hydrophobic channel was mutated to smaller uncharged amino acids
(Arg120Ala, Arg120Gln, and Arg120Asn). Mutation of residues located
within the active site of COX-2 that define a `side pocket'
(Tyr355Ala, Val523Ile, IHI) of the main channel had a greater effect on
the dissociation rate than the association rate. These mutations, which
modified the shape of and access to the `side pocket', affected the
binding affinity of [3H]valdecoxib more than that of
[3H]celecoxib. These binding studies provide direct
insight into the properties and binding constants of celecoxib and
valdecoxib to COX-2.
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