Table 1 Comparison of in vitro effects of compounds with different pKa values.

From: pKa of opioid ligands as a discriminating factor for side effects

Drug

Calculated pKa

Experimentally obtained pKa

MOR binding (IC50 nM)

in HEK 293 cells

Maximum [35S]-GTPγS binding at 100 μM agonist

(% of baseline)

EC50 of [35S]-GTPγS binding

(nM)

pH 6.5

pH 7.4

pH 6.5

pH 7.4

pH 6.5

pH 7.4

Fentanyl

9.113

8.44 ± 0.0518

6.9 ± 1.115

4.8 ± 0.715

157.7 ± 9.1

144.1 ± 2.9

125.5 ± 20.63

96.1 ± 16.63

FF6

n/a

7.94 ± 0.01

2.3

CI: −0.1 to 4.6

2.0

CI: −0.002 to 4.1

157.3 ± 3.9

151.3 ± 4.4

22.8

CI: −6.8 to 52.4

17.9

CI: 1 to 34.8

FF3

6.013

7.22 ± 0.013

7.1 ± 1.43

48.3 ± 6.4***3

129.9 ± 11.6

164.9 ± 21.4

47.2 ± 24.23

231.4 ± 47.9**3

NFEPP

6.8315

6.82 ± 0.0615

18.3 ± 3.315

81.9 ± 22.1*15

149.4 ± 10.6

137.7 ± 2.4

131.4 ± 29.8

337.7 ± 166.1

  1. Data are represented as mean ± SEM (for normally distributed data) or mean with 95% confidence intervals (for non-normally distributed data). N.S., not significant P > 0.05, *P < 0.05; **P < 0.01; ***P < 0.0001; Mann-Whitney or unpaired t-test (n = 6–8); n/a: n.
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