A 38-year-old man presents to the clinic for a follow-up evaluation of worsening fatigue, … — USMLE Step 3

A $38$ -year-old man presents to the clinic for a follow-up evaluation of worsening fatigue, a $6\text{-kg}$ ( $13.2\text{-lb}$ ) weight gain, and easy bruising over the past $3$ months. He also reports new-onset muscle weakness when climbing stairs. His medical history is significant for HIV, diagnosed $4$ years ago, and severe seasonal allergic rhinitis. His current medications include a single-tablet antiretroviral regimen consisting of elvitegravir, cobicistat, emtricitabine, and tenofovir alafenamide. Four months ago, he was prescribed a fluticasone nasal spray for his allergic rhinitis, which he uses twice daily.

Vital signs are temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure $158/96 \text{ mmHg}$ , pulse $82\text{/min}$ , and respirations $14\text{/min}$ . BMI is $31 \text{ kg/m}^2$ . Physical examination reveals a plethoric face, prominent supraclavicular fat pads, and wide violaceous striae on the lower abdomen.

Laboratory studies:
Sodium: $142 \text{ mEq/L}$
Potassium: $3.8 \text{ mEq/L}$
Fasting glucose: $135 \text{ mg/dL}$
Morning serum cortisol: $1.2 \text{ \mu g/dL}$ (Normal: $6\text{--}23 \text{ \mu g/dL}$ )
Plasma ACTH: $< 5 \text{ pg/mL}$ (Normal: $10\text{--}60 \text{ pg/mL}$ )
HIV viral load: Undetectable
CD4 T-lymphocyte count: $620\text{/mm}^3$

What is the most appropriate next step in the pharmacologic management of this patient?

Transition the antiretroviral therapy to a bictegravir-based regimen and initiate a glucocorticoid taperAnswer
B
Switch the fluticasone nasal spray to budesonide nasal spray and maintain current antiretrovirals
C
Immediately discontinue the fluticasone nasal spray and monitor for symptom resolution
D
Discontinue cobicistat from the antiretroviral regimen to allow normal fluticasone metabolism
E
Initiate oral ketoconazole therapy to inhibit adrenal steroidogenesis

Answer

Transition the antiretroviral therapy to a bictegravir-based regimen and initiate a glucocorticoid taper

Cobicistat is a potent pharmacokinetic enhancer that strongly inhibits the cytochrome P450 3A4 (CYP3A4) enzyme. Fluticasone is a synthetic glucocorticoid that relies on extensive first-pass metabolism by CYP3A4. Concomitant administration prevents fluticasone clearance, leading to massive systemic accumulation. This causes iatrogenic Cushing syndrome and profound negative feedback on the hypothalamic-pituitary-adrenal (HPA) axis, evidenced by the suppressed ACTH and endogenous cortisol. The definitive management is to remove the CYP3A4 inhibitor by transitioning the patient to an effective antiretroviral regimen that does not require boosting, such as an unboosted integrase inhibitor (e.g., a bictegravir-based regimen). However, because the fluticasone will rapidly clear once the CYP3A4 inhibition is removed, and the patient's HPA axis remains suppressed, a systemic glucocorticoid taper must be initiated simultaneously to prevent an acute, life-threatening adrenal crisis.

Step-by-Step Solution

Identify the clinical syndrome based on the physical exam and metabolic findings.

The patient's weight gain, plethora, striae, weakness, and hyperglycemia indicate Cushing syndrome.

Recognizing the phenotype is the first step in determining the underlying etiology.

Interpret the endocrinology laboratory results.

The low morning cortisol and profoundly suppressed ACTH confirm that the Cushing syndrome is iatrogenic (exogenous), caused by the fluticasone nasal spray.

Endogenous Cushing syndrome would present with elevated cortisol and either elevated (pituitary/ectopic) or suppressed (adrenal) ACTH. Suppressed values for both indicate exogenous glucocorticoid excess.

Identify the underlying pharmacokinetic drug interaction.

Cobicistat is a potent CYP3A4 inhibitor, which prevents the normal hepatic clearance of fluticasone (a CYP3A4 substrate), leading to massive systemic accumulation.

Fluticasone normally has low systemic bioavailability, but CYP3A4 inhibition drastically increases its systemic exposure, causing iatrogenic Cushing syndrome.

Determine the safest pharmacologic intervention.

The CYP3A4 inhibitor must be removed by switching to an unboosted HIV regimen (e.g., bictegravir). Simultaneously, a glucocorticoid taper must be initiated.

Removing the interaction will cause fluticasone levels to drop rapidly. Because the HPA axis is suppressed, failing to provide exogenous glucocorticoid coverage during this transition will precipitate a life-threatening adrenal crisis.

Key Concept

Management of pharmacokinetic interactions between potent CYP3A4 inhibitors (cobicistat/ritonavir) and CYP3A4 substrates (fluticasone), including mitigating the risk of adrenal crisis upon withdrawal.

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