A 32-year-old man presents to the emergency department with severe diffuse muscle cramps, … — USMLE Step 2 CK

A 32-year-old man presents to the emergency department with severe diffuse muscle cramps, nausea, vomiting, and profound restlessness. He has a 5-year history of intravenous heroin use. He reports that he last injected heroin 12 hours ago. Determined to quit, he took a sublingual film given to him by a friend who is currently in a medication-assisted treatment program. Within 30 minutes of taking the film, his symptoms rapidly worsened. Vital signs show a temperature of 37.2°C (99.0°F), blood pressure of 152/94 mm Hg, heart rate of 118/min, and respiratory rate of 18/min. Physical examination reveals mydriasis, rhinorrhea, excessive lacrimation, and piloerection. Bowel sounds are hyperactive. Which of the following pharmacodynamic properties of the principal active medication in this sublingual film best explains the rapid exacerbation of this patient's symptoms?

High affinity and low intrinsic activity at the mu-opioid receptorAnswer
B
High affinity and zero intrinsic activity at the mu-opioid receptor
C
Full agonism at the mu-opioid receptor and NMDA receptor antagonism
D
Agonism at central presynaptic alpha-2 adrenergic receptors
E
Agonism at the kappa-opioid receptor and antagonism at the mu-opioid receptor

Answer

High affinity and low intrinsic activity at the mu-opioid receptor

The correct answer accurately describes the pharmacodynamics of buprenorphine. The patient is experiencing precipitated opioid withdrawal. He ingested a sublingual medication used for opioid use disorder, which is typically a combination film of buprenorphine and naloxone. Buprenorphine is a partial mu-opioid receptor agonist. This means it has very high binding affinity for the receptor, but low intrinsic activity compared to full agonists like heroin. Because of its exceptionally high affinity, buprenorphine tightly binds to the mu-opioid receptors, effectively displacing any residual heroin. However, because it has low intrinsic activity, it provides significantly less receptor activation than the patient's current tolerance level requires. This sudden drop in opioid receptor stimulation rapidly plunges the dependent patient into severe acute withdrawal.

Step-by-Step Solution

Identify the patient's clinical syndrome based on history and physical exam findings.

The patient is in acute opioid withdrawal, evidenced by mydriasis, piloerection, rhinorrhea, vomiting, and sympathetic overactivity.

Recognizing the toxidrome is the first step in determining the underlying etiology of the rapid clinical deterioration.

Determine the likely composition of the ingested sublingual film.

Sublingual films given in medication-assisted treatment (MAT) programs for opioid use disorder typically contain a combination of buprenorphine and naloxone.

Identifying the active drugs is necessary to understand the pharmacodynamic mechanism.

Evaluate the pharmacokinetics of the drug components when administered sublingually.

Sublingual buprenorphine is well-absorbed systemically. Sublingual naloxone has extremely poor bioavailability (<10%) and remains largely inert in the system.

This rules out naloxone as the cause of the precipitated withdrawal, as it is only active if the film is illicitly dissolved and injected intravenously.

Apply the pharmacodynamic profile of the active systemic drug to the patient's physiology.

Buprenorphine is a partial mu-opioid receptor agonist. It has higher binding affinity than heroin/methadone but lower intrinsic activity.

By competitively displacing residual heroin from the receptors without providing equivalent activation, buprenorphine causes a rapid, net deficit in opioid signaling, leading to precipitated withdrawal.

Key Concept

Precipitated opioid withdrawal due to buprenorphine partial agonism

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