Case Study 1: When “Normal” Drug Metabolism Did Not Explain Antidepressant Failure

Reason to come to PPI: Two antidepressants failed.

A patient came to Personalized Prescribing after trying two different SSRI antidepressants.

Each medication was taken for approximately eight weeks, giving enough time to evaluate whether it was helping. Unfortunately, neither medication produced meaningful symptom improvement. Each medication was then gradually tapered and discontinued under medical supervision.

After two unsuccessful treatment attempts, the patient wanted to understand: Why were the antidepressants not working?

PPI’s Analysis

The First Question: Was the Patient Metabolizing the Medication Properly?

Many pharmacogenomic tests focus mainly on liver enzymes, such as:

• CYP2D6
• CYP2C19 These enzymes help determine how quickly the body processes many antidepressants.

PPI testing showed that the patient’s relevant medication-processing enzymes were functioning normally. In simple terms:

1. The medications were getting into the body
2. The body was processing them normally
3. There was no major genetic metabolism problem to explain the two treatment failures.

A pharmacogenomic test that looked only at drug metabolism might therefore have reported little reason to reconsider the medication mechanism.

PPI Looked Beyond Drug Metabolism

PPI also examined genes involved in how the serotonin system functions and how antidepressants interact with it. Two findings were particularly interesting.

• HTR1A — The Serotonin “Brake” HTR1A helps control a serotonin receptor that can act as a feedback brake on serotonin release.

The patient’s genetic profile was associated with stronger activity of this serotonin autoreceptor.

A simple way to think about it is:

1. SSRI blocks serotonin reuptake
2. Serotonin begins to increase
3. The serotonin autoreceptor senses the increase
4. The brake reduces further serotonin release

In someone with stronger autoreceptor activity, this feedback system could potentially make it more difficult for a conventional SSRI mechanism alone to produce the desired effect.

SLC6A4 — The Serotonin Transporter

PPI also found a genetic pattern associated with somewhat lower serotonin transporter activity.

The serotonin transporter is important because it is the main target of traditional SSRIs.

SSRIs work mainly by: Blocking the serotonin transporter

If a person’s serotonin transporter system functions differently, the biological effect of simply blocking that transporter may also differ.

Neither HTR1A nor SLC6A4 alone can prove why an antidepressant failed. However, when considered together with two previous SSRI failures and otherwise normal drug metabolism, these findings provided the pharmacist with additional information about the patient’s serotonin pathway.

A Different Medication Strategy

Rather than recommending another conventional SSRI, the PPI pharmacist considered a medication with a broader serotonin mechanism.

Vortioxetine was recommended for discussion with the treating clinician.

Vortioxetine still affects the serotonin transporter, but unlike a conventional SSRI, it also acts directly on several serotonin receptors, including the 5-HT1A receptor.

The recommendation was therefore based on the patient’s:

• Previous medication failures
• Normal pharmacokinetic results
• Serotonin transporter genetics
• Serotonin receptor genetics A broader medication strategy The final prescribing decision remained with the patient’s treating healthcare professional.

What Happened Next?

PPI followed up with the patient approximately four months later. The patient reported that the new medication was working and that they were experiencing:

• Meaningful symptom relief
• A significant improvement in how they felt.

Why This Case Matters

If this patient had received a pharmacogenomic test that examined only liver metabolism genes, the result might simply have shown: “Normal metabolism.”

That information is useful—but in this case, it would not have explained why two adequately tried SSRIs had failed.

PPI’s broader approach considered both:

• Pharmacokinetics: Can the body process the medication properly?
• Pharmacodynamics- How might the biological system targeted by the medication respond? This allowed the pharmacist to ask a different question: The medication is reaching the patient normally—but is its mechanism the right fit for this person’s biology?

The Takeaway

Medication response is more complicated than how quickly the liver breaks down a drug.

A more complete assessment may consider:

• Drug metabolism
• Transporters
• Receptors
• Previous treatment response
• Current symptoms A more personalized medication decision

Important Note

This is an individual case and does not mean that HTR1A or SLC6A4 can independently predict whether an SSRI will succeed or fail. Evidence for many pharmacodynamic genes is still developing. PPI uses these findings as supporting information within a broader clinical and genetic assessment, rather than as stand-alone prescribing rules.