Biochemical Misdiagnosis of Pheochromocytoma in Patients Treated With Sulfasalazine FREE

To the Editor: The excessive secretion of catecholamines by pheochromocytoma can have serious medical consequences.¹ Adrenal incidentaloma is becoming an important public health challenge in aging populations.² Initial evaluation for possible pheochromocytoma or adrenal incidentaloma generally includes measurements of fractionated metanephrines in urine, plasma, or both.^1,3 We report 3 cases of biochemical misdiagnosis of pheochromocytoma in patients being treated with sulfasalazine.

Between 2006 and 2009, 3 women were admitted to the hospital with suspected pheochromocytoma (in 2) or adrenal incidentaloma (in 1) (Table). All were receiving sulfasalazine (1500 mg or 2000 mg daily) for either rheumatoid arthritis or ankylosing spondylitis. The 24-hour urinary normetanephrine (UNM) levels, measured several times by high-performance liquid chromatography with electrochemical detection (HPLC-EC), were extremely high (mean [SD], 12 064 [7993], 12 708 [4220], and 10 946 [1435] μg/d, respectively; normal range, 105-600 μg/d). Urinary and plasma metanephrine and norepinephrine levels remained normal. Anatomical as well as functional imaging did not show any evidence of a pheochromocytoma or paraganglioma (Table). After a false-positive result in UNM assay due to sulfasalazine was suspected, sulfasalazine treatment was stopped. Approximately 1 month later, UNM levels had returned to normal in all 3 patients (310, 501, and 249 μg/d, respectively). The first patient decided to take sulfasalazine again, and UNM levels checked twice 1 month later returned to high levels (mean [SD], 17 209 [6231] μg/d).

To assess for interference by sulfasalazine, the UNM levels of the patients were analyzed by mass spectrometry (LCQ Finnigan MAT, San Jose, California). The results with the UNM levels obtained by both techniques were compared in 3 patients with pheochromocytoma and 3 healthy control participants. Because sulfasalazine is a prodrug, to assess whether interference could be due to 5-aminosalicylic acid (mesalamine, an active breakdown product of sulfasalazine formed in the gut), chromatograms of pure samples of sulfasalazine (S0883; Sigma, St Louis, Missouri), mesalamine (A3537; Sigma), and normetanephrine (Sigma) were directly compared for coelution.

The institutional review board of the University Hospital Rangueil approved this study, and all participants provided written informed consent.

Mean (SD) UNM concentration measured by HPLC-EC was 6477 (3184) μg/L in patients using sulfasalazine, 3096 (1713) μg/L in patients with pheochromocytoma, and 300 (134) μg/L in healthy controls. Mean (SD) UNM abundance measured by mass spectrometer was 2.34 × 10⁵ (7.39 × 10⁴) for sulfasalazine patients, 1.99 × 10⁶ (1.47 × 10⁶) for patients with pheochromocytoma, and 2.08 × 10⁵ (9.81 × 10⁴) for healthy controls. By HPLC-EC, the patients treated with sulfasalazine had UNM values 2159% of control values, whereas the pheochromocytoma patients had UNM values 1032% of control values. A urine chromatogram from patient 1 compared with a standard normetanephrine sample is shown in Figure A. However, by mass spectrometry, the patients treated with sulfasalazine had only 113% of control values, whereas patients with pheochromocytoma had 958% of control values. Pure mesalamine and pure sulfasalazine did not interfere with normetanephrine (Figure B).

The mass spectrometry analysis demonstrates that the urinary HPLC-EC UMN assay routinely used in the clinic may experience interference by a molecule released into the urine during sulfasalazine treatment. This interference has been observed in patients treated with mesalamine.^4- 5 However, because pure mesalamine did not coelute with normetanephrine in HPLC-EC (Figure B), the interfering compound in urine of sulfasalazine-treated patients might be a metabolite of mesalamine.

Although plasma metanephrine testing may be more specific,¹ urinary testing is included in guidelines and remains very common globally.³ Our findings would support using plasma metanephrines instead of urinary metanephrines to detect pheochromocytoma, particularly when patients use medications that could interfere with the urinary assay.^1,6 Recognition of sulfasalazine as a medication that interferes with assays for UNM can avoid the unnecessary exposure of patients to radiation and the cost of additional investigation for neuroendocrine tumors.