· Equipotent doses
· Time course
· Absorption
· Distribution
· Metabolism
· Excretion
|
|
Morphine |
Oxycodone |
|
IV |
10mg |
10mg |
|
PO |
30mg |
20mg |
|
|
Morphine |
Oxycodone |
|
Onset (time to 80%) (mins) |
10 mins |
2-5mins |
|
Peak effect (mins) |
30 mins |
? faster than morphine |
|
Duration (hours) |
3h |
3h |
|
|
Morphine |
Oxycodone: |
|
Oral bioav |
30-50% |
75% |
|
|
Morphine |
Oxycodone |
|
Lipid solubility |
1 |
1 |
|
pKa (weak bases) |
8.0 |
8.5 |
|
% unionised at pH 7.4 |
23% |
<10% |
|
Active uptake into CNS |
N |
Y* |
|
%Plasma protein bound |
35% |
45% |
|
VDC (L/kg) |
|
|
|
VDSS (L/kg) |
3.5 |
2.5 |
|
t1/2ke0 (mins) |
17 |
? faster |
|
t1/2α (mins) |
|
|
*Active uptake into CNS may account for oxycodone’s apparent faster onset despite similar size, lipid solubility and ionised fraction
|
|
Morphine |
Oxycodone |
|
Organ |
Liver |
Liver |
|
Mode |
Phase 2(less affected in liver failure) |
Phase 1 then 2 |
|
Clearance (mL/kg/min) |
16 |
13 |
|
t1/2β (mins) |
170 |
200 |
|
Metabolites |
(1) Morphine-3-glucuronide (60%) -Neurotoxic: seizures, confusion -Inactive as analgesic (2) Morphine-6-glucuronide (10%) -Active: 14x analgesic potency (3) Normorphine (30%) -Inactive |
(1) 3A4 -> noroxycodone (most) (2) 2D6 -> oxymorphone (15%) -14x analgesic potency (3) 6α and 6β-oxycodol (10%) (4) Oxycodone-n-oxide |
|
|
Morphine |
Oxycodone |
|
Route |
Metabolites -> urine* Minimal morphine -> urine |
Metabolites -> urine* 10% oxycodone -> urine |
*Renal failure results in accumulation of active metabolites for both drugs, and oxycodone itself
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