Table 4. Plasma Stability of Enkephalin and Its Analogues.
| compd | half-life (min)a | 95% CI | degradation products (time for first appearance)b |
|---|---|---|---|
| Leu-enkephalin | 4.6 | [4.3–5.1] | Gly-Gly-Phe-Leu (5 min) |
| Phe-Leu (10 min) | |||
| 4 | 3.3 | [3.0–3.9] | Gly-Gly-Phe-OLeu (5 min) |
| Phe-OLeu (5 min) | |||
| Tyr-Gly-Gly-Phe (5 min) | |||
| Gly-Gly-Phe (20 min) | |||
| 8 | 10.7 | [8.7–12.7] | Gly-Gly-Phe-NMeLeu (5 min) |
| Phe-NMeLeu (20 min) |
a
Half-life: The stability of each compound was determined using an Agilent 1100 series Symmetry C-18 HPLC column, 150 mm × 4.6 mm, 5 μm, heated at 30 °C, flow, 1.2 mL/min, start with 0.1% TFA in water then 0 to 75% acetonitrile in 20 min, UV detection at 223 nm.
b
Degradation products: Degradation products in diluted rat plasma were identified by LC-MS analysis.