Protriptyline, desipramine dose related neurotransmitter increase in % vs venlafaxine 375 mg
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Dose-Related Neurotransmitter Reuptake Inhibition: Protriptyline & Desipramine vs. Venlafaxine 375 mg
Overview: Transporter Affinity (In Vitro Ki)
This is the foundation for understanding dose-related effects. Lower Ki = higher affinity.
| Drug | SERT Ki (nM) | NET Ki (nM) | NE:5-HT selectivity |
|---|---|---|---|
| Desipramine | ~18 | ~1 | ~18x NET-selective |
| Protriptyline | ~19 | ~1.4 | ~14x NET-selective |
| Venlafaxine | ~82 | ~2,480 | ~30x SERT-selective |
Desipramine and protriptyline are highly NET-preferring secondary amine TCAs. Venlafaxine is the inverse - heavily SERT-preferring.
Venlafaxine: Dose-Dependent In Vivo Reuptake Inhibition (%)
This is well-characterized from ex vivo blood assay studies (Owens et al. 2008; Debonnel et al. 2007):
| Venlafaxine Dose | SERT Inhibition (5-HT) | NET Inhibition (NE) |
|---|---|---|
| 75 mg/d | ~80-85% | ~25% |
| 150 mg/d | ~85-90% | ~45% (but many patients still primarily SSRI-like) |
| 225 mg/d | >90% | ~50% |
| 300 mg/d | >95% | ~55% |
| 375 mg/d | >95% (plateau) | ~60% |
Key points from Blier et al. (2022, PMC9017767):
- At 225 mg/d, 8/10 patients had >90% whole-blood 5-HT depletion (a SERT proxy)
- At 300-375 mg/d, depletion exceeded 95%
- SERT inhibition plateaus early; NET inhibition continues climbing dose-dependently
- At 375 mg/d, NET inhibition reaches approximately 60%
A separate study (Owens et al. 2008, Nature Neuropsychopharmacology) confirmed venlafaxine XR titrated to max (375 mg/d) achieved:
- SERT: ~85% (SD ±10)
- NET: ~60% (SD ±13)
Desipramine: In Vivo NE/5-HT Reuptake Inhibition
Desipramine is the prototypical selective NET inhibitor among TCAs.
| Parameter | Value |
|---|---|
| NET inhibition (therapeutic range 75-200 mg/d) | ~60% (constant, non-dose-dependent) |
| SERT inhibition (same range) | <20% |
| NET occupancy pattern | Flat - reaches ~60% and stays there across dose range |
This is a remarkable finding from nortriptyline/desipramine PET/occupancy studies: unlike venlafaxine, desipramine does not show a dose-response relationship for NET occupancy once in therapeutic range. NET occupancy stays ~60% from 75 to 200 mg/d.
Protriptyline: In Vivo Profile
Protriptyline shares desipramine's pharmacological profile as a secondary amine TCA with marked NET selectivity:
| Parameter | Value |
|---|---|
| NET selectivity | ~14x over SERT (Ki NET ~1.4 nM vs SERT ~19 nM) |
| Functional profile | Highly NE-selective, minimal 5-HT |
| NET inhibition (therapeutic doses 15-60 mg/d) | Estimated ~60%, similar to desipramine |
| SERT inhibition | Negligible (<20%) |
Direct in vivo % data for protriptyline specifically is sparse in the published literature - most data extrapolates from desipramine given the near-identical binding profile and class pharmacology.
Side-by-Side Comparison at Comparable Clinical Doses
| Drug | Dose | NE (NET) Inhibition | 5-HT (SERT) Inhibition | DA (DAT) |
|---|---|---|---|---|
| Protriptyline | 15-60 mg/d | ~60% | <15% | Negligible |
| Desipramine | 75-200 mg/d | ~60% | <20% | Negligible |
| Venlafaxine | 75 mg/d | ~25% | ~80-85% | Negligible |
| Venlafaxine | 150 mg/d | ~45% | ~85-90% | Negligible |
| Venlafaxine | 225 mg/d | ~50% | >90% | Negligible |
| Venlafaxine | 375 mg/d | ~60% | >95% | Weak (~5%) |
Clinical Takeaway
At its maximum dose of 375 mg/d, venlafaxine finally matches the NET inhibition of desipramine and protriptyline (~60%), while also delivering maximal SERT inhibition (>95%). By contrast:
- Desipramine and protriptyline are pure NE-dominant agents with negligible serotonergic activity at any dose
- Venlafaxine 75-150 mg/d acts essentially as an SSRI for most patients - NET inhibition is modest
- The "SNRI" character of venlafaxine only becomes clinically meaningful at 225 mg/d and above
- Venlafaxine does not surpass desipramine/protriptyline in NE reuptake inhibition - it approximates them at 375 mg/d, but adds far greater 5-HT activity
Sources: Owens et al. (2008) Neuropsychopharmacology; Blier et al. (2022) Int J Neuropsychopharmacol PMC9017767; Debonnel et al. (2007); Richelson (1994) receptor binding data.
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