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11C-Metomidate PET-CT versus adrenal vein sampling to subtype primary aldosteronism: a prospective clinical trial

Puar, Troy H.a,b; Khoo, Chin Mengc,d; Tan, Colin Jingxiane; Tong, Aaron Kian Tif; Tan, Michael Chien Shengg; Teo, Ada Ee Derh; Ng, Keng Sini,j; Wong, Kang Mini; Reilhac, Anthonink; O’Doherty, Jimk; Gomez-Sanchez, Celso E.l; Kek, Peng Chinm; Yee, Szemenn; Tan, Alvin W.K.o; Chuah, Matthew Bingfengp; Lee, Daphne Hui Minq; Wang, Kuo Wengr; Zheng, Charles Qishib,s; Shi, Lumingb,s; Robins, Edward Georgek,t; Foo, Roger Sik Yind,u,v

Author Information
doi: 10.1097/HJH.0000000000003132

Abstract

INTRODUCTION

Primary aldosteronism affects 5–20% of all patients with hypertension, making it the most treatable cause of hypertension [1]. However, many patients remain undiagnosed due to challenging steps in the diagnostic algorithm [2]. Although primary aldosteronism can be treated with mineralocorticoid-receptor antagonists, unilateral primary aldosteronism is curable with laparoscopic adrenalectomy [3]. However, identifying unilateral primary aldosteronism is particularly challenging, leading to several notable consequences. First, patients with primary aldosteronism, compared with those with essential hypertension, are at greater risk of cardiovascular disease [4], renal failure and suffer from poorer quality of life [5]. Second, surgery offers better outcomes to long-term medications [6]. Finally, these patients miss the opportunity for cure of primary aldosteronism and hypertension.

Computed tomography (CT) of the adrenals is unreliable as 6% of the population may harbour a non-functional adenoma, while small functional adenomas may be missed [7]. Thus, adrenal vein sampling (AVS) is required in almost all patients for subtyping primary aldosteronism [3,8]. In a randomized-controlled trial comparing CT versus AVS for subtyping, patients who underwent surgery in both arms had similar blood pressure (BP) improvements [9]. However, a higher proportion of patients in the CT-arm had failure of biochemical cure compared with AVS-arm. Although this difference was not statistically significant, this study was not adequately powered to assess for this. The Primary Aldosteronism Surgery Outcomes (PASO) consensus has since been adopted to assess biochemical and clinical outcomes post-surgery [10]. In a retrospective multicentre cohort study, CT-guided surgery led to a significantly lower rate of complete biochemical cure compared with AVS (80 versus 93%) [11]. Although this confirmed that AVS is superior to CT, 7% of patients with AVS lateralization did not have biochemical cure post-surgery [10,11]. AVS is also invasive and technically-challenging, with success rates of nearly 30–60% in some centres [12], and nearly 80% in specialized centres [13].

Functional imaging offers an alternative to AVS. 131I-iodomethyl-norcholesterol (NP-59) scintigraphy was previously used, but it could only detect aldosterone-producing adenomas (APA) greater than 1.5 cm [14]. Metomidate inhibits adrenal steroid enzymes 11-beta-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2), and can be C11H-labelled as a PET radiotracer [15]. Pre-treatment with dexamethasone increases the specificity of 11C-Metomidate for CYP11B2 [16]. Coupled with high-resolution CT, 11C-Metomidate PET-CT can detect functional APA less than 1 cm. In a previous study, 11C-Metomidate PET-CT had a specificity and sensitivity of 87 and 76%, respectively, to detect unilateral primary aldosteronism, using AVS as the reference [16]. However, as biochemical cure of primary aldosteronism post-surgery is a better determinant of unilateral primary aldosteronism, we assessed the accuracy of PET versus AVS using biochemical cure post-surgery as the reference.

DESIGN AND METHODS

Study design and participants

We conducted a single-arm prospective clinical trial using both 11C-Metomidate PET-CT and AVS to subtype primary aldosteronism. Patients were recruited from Changi General Hospital and National University Hospital, Singapore. All patients provided written informed consent prior to participation. The study was approved by local ethics committee, Health Science Authority (Singapore), and registered with Clinicaltrials.gov (NCT03990701).

We prospectively recruited 25 consecutive patients with primary aldosteronism, who were keen for surgery if unilateral primary aldosteronism was diagnosed. Inclusion criteria were ages 21–70 years, and confirmed primary aldosteronism according to the Endocrine Society guidelines (further details in supplement, https://links.lww.com/HJH/B919) [3]. We excluded patients unable to provide informed consent, those with chronic renal failure stage 3b or greater, terminal medical condition or contraindications to isotope scanning, for example pregnancy, or claustrophobia.

Subtype tests

All patients underwent thin-slice CT of the adrenals. CT diagnosis was based on initial radiologist report of the presence of adrenal nodule. All patients underwent AVS with sequential cannulation of both adrenal veins under continuous cosyntropin infusion started at least 30 min before sampling. AVS was successful if cortisol levels in both adrenal veins were five times or greater than peripheral vein. Lateralization ratio was determined by aldosterone-to-cortisol ratio of the higher adrenal vein (dominant side) divided by corresponding ratio of the contralateral side. Patients had unilateral primary aldosteronism if lateralization ratio was four and above.

Patients received an injected dose of 150–300 MBq of 11C-Metomidate with PET acquisition thereafter (details in supplement, https://links.lww.com/HJH/B919). All patients received 0.5 mg dexamethasone six hourly 72 h prior to PET [16]. Patients with diabetes mellitus were required to have glycosylated haemoglobin (HbA1C) less than 10% before dexamethasone ingestion.

11C-Metomidate PET-CT scans were analysed independently by two nuclear medicine physicians (C.J.T. and A.K.T.) blinded to clinical data and AVS results. Any disagreements were decided by mutual consensus. PET Lateralization was defined as ‘clear lateralization’ if SUVmax in the nodule was more than 1.25 times compared with contralateral adrenal gland [16]. ‘Likely lateralization’ was defined as SUVmax in the nodule more than 1.0 and less than 1.25 times greater than contralateral gland, or SUVmax in one adrenal gland was more than 1.25 times greater than contralateral gland in absence of a nodule. SUVmax ratio was calculated by SUVmax of the nodule, or adrenal gland with high uptake in absence of a nodule, divided by SUVmax of contralateral gland.

Patient allocation and blinding

All patients underwent AVS and PET in random order, which were conducted at least 5 days apart. Interpretation of AVS and PET were done independently, and reviewers were blinded to the other test. Subsequently, all test results were discussed in a multidisciplinary meeting comprising endocrinologists, radiologists and surgeons, to decide on the final diagnosis and treatment. In four cases of discordant AVS and PET results, decision was made to repeat AVS despite a previous successful AVS (details in supplement, https://links.lww.com/HJH/B919). The first successful AVS for all patients was used for data analysis.

Outcomes

At least 6 months post-treatment, all patients were reassessed for biochemical and clinical outcomes according to the PASO consensus [10]. Clinic BP was done after 5 min of rest, and the average of at least two BP readings used. Use of antihypertensive medications were recorded as defined daily dosage (DDD) (https://www.whocc.no/atc_ddd_index/). The primary outcome was the accuracy of each subtype test compared with biochemical cure of primary aldosteronism post-surgery (patients with unilateral primary aldosteronism were required to have complete biochemical success post-surgery). Secondary outcome was the accuracy of each diagnostic test compared to final clinical subtype diagnosis (made at the multidisciplinary adrenal meeting).

Statistical analysis and sample size calculation

This study was designed to have a power of 80% to detect a sensitivity of 0.8 comparing to an indifference diagnosis (sensitivity of 0.5) using one-sided test at the significance level of 0.05. Statistical analysis was performed using SPSS Version 24.0 (IBM Corp., Armonk, New York, USA). Continuous variables were expressed as median (IQR) and compared using the Mann--Whitney U-test. Chi-square test was used for categorical variables. Statistical significance was set at P value less than 0.05.

For the primary endpoint, the biochemical cure rate of primary aldosteronism after surgery were calculated for both 11C–Metomidate PET–CT and AVS. For the secondary endpoint, measuring the diagnostic accuracy of both methods, the sensitivity, specificity were reported.

Details on histological and immunohistochemistry analysis are included in the supplement, https://links.lww.com/HJH/B919.

RESULTS

We prospectively approached 28 consecutive patients for the study, and 25 patients were recruited, with no withdrawals (Fig. 1). Eleven patients (44%) were women, and the median age was 49 (40–55) years (Table 1). Median duration of hypertension was 7 [4–16] years, and patients were on 2.0 (1.2–4.0) DDD of antihypertensive medications. 23 patients were confirmed with primary aldosteronism after intravenous saline-loading test, with median post-saline PAC 443 (319–789) pmol/l, while two patients had baseline PAC more than 550pmol/l, undetectable renin and spontaneous hypokalaemia.

F1
FIGURE 1:
Results of AVS, 11C-Metomidate PET-CT and treatment of all 25 patients. AVS, adrenal vein sampling; CT, computed tomography; PET, 11C-Metomidate PET-CT imaging.
TABLE 1 - Characteristics of all 25 patients recruited in study treated with either surgery (n = 20) or medications (n = 5)
Total (n = 25) Surgery (n = 20) Medications (n = 5)
Baseline Baseline P a Post-treatment P b Baseline Post-treatment P b
Age (years) 50.0 (42.1–56.5) 50.1 (40.1–55.2) 0.64 46.7 (42.4–64.0)
Sex, Female / Male 11 / 14 9 / 11 0.84 2 / 3
BMI (kg/m2) 26.4 (22.2–29.2) 25.6 (21.5–29.5) 0.19 27.1 (26.0–32.1)
Duration of hypertension (years) 7 (4–16) 7 (4–17) 1.0 8 (2–19)
SBP (mmHg) 142.5 (136.5–148.9) 142.7 (137.9–150.6) 0.30 133.5 (128.1–138.6) 0.029 139.2 (130.4–145.6) 130.7 (119.3–143.5) 0.18
DBP (mmHg) 86.8 (80.3–92.2) 87.1 (79.1–92.5) 0.72 85.8 (83.2–89.1) 0.84 86.8 (86.4–90.2) 79.0 (72.2–86.8) 0.77
Antihypertensive medications, DDD 2.0 (1.2–4.0) 2.3 (1.1–5.0) 0.49 0.8 (0–2) 0.001 2.0 (1.5–2.9) 2.9 (2.4–4.4) 0.030
Antihypertensive medications, number 2 (1–2) 2 (1–2) 0.34 1 (0–2) <0.001 1 (1–2) 2 (1.5–4) 0.033
Serum potassium (mmol/l) 3.5 (3.3–3.8) 3.5 (3.1–3.9) 0.82 4.1 (4.0–4.4) <0.001 3.5 (3.5–3.7) 3.9 (3.4–4.1) 0.29
Creatinine (μmol/l) 66 (56–84) 67 (57–85) 0.72 78.5 (67–112·8) 0.001 66 (47–94) 73 (51–91) 0.34
eGFR (ml/min per 1.73 m2) 102 (90–113) 101 (90–106) 0.72 112 (76–116)
PAC (pmol/l) 803 (557–1169) 914 (665–1379) 0.042 111 (111–163) <0.001 471 (402–875) 402 (319–568) 0.72
PRA (ng/ml/h) 0.6 (0.6–0.8) 0.6 (0.6–0.6) 0.272 0.8 (0.6–0.9) 0.65 (0.6–2.1) 0.50
Post-saline infusion PAC (pmol/l) (n = 23) 499 (388–1072) 571 (410–1163) 0.015 349 (299–416)
ARR (pmol/l) per ng/ml/h 1247 (717–1795) 1294 (867–2285) 0.06 89 (39–139) <0.001 693 (488–1205) 510 (310–681) 0.25
CT findings
 Unilateral adenoma 19 15 0.46 4
 Bilateral adenoma 1 1 0
 Bilateral normal adrenals 5 4 1
Presence of diabetes mellitus 8 (32%) 5 (25%) 0.13 3 (60%)
Presence of hyperlipidaemia 9 (36%) 7 (35%) 0.84 2 (40%)
SUVmax ratio 1.40 (1.10–1.78) 1.41 (1.12–1.84) 0.78 1.14 (0.96–1.98)
AVS lateralization ratio 9.6 (3.4–27.6) 13.0 (3.9–38.3) 0.024 3.4 (1.8–4.2)
Data are presented as n (%), median (IQR).ARR, aldosterone-to-renin ratio; AVS, adrenal vein sampling; BP, blood pressure; CT, computed tomography; DDD, defined daily dose according to WHO (https://www.whocc.no/atc_ddd_index/); eGFR, estimated glomerular filtration ratio; PAC, plasma aldosterone concentration; PRA, plasma renin activity; SUVmax, maximal standardized uptake value.
aBaseline surgery vs. baseline medical therapy.
bBaseline vs. post-treatment.

Diagnosis of unilateral and bilateral primary aldosteronism and treatment

All 25 (100%) patients had successful AVS, and 11C-Metomidate PET-CT performed. Twenty-two patients were diagnosed with unilateral primary aldosteronism (lateralization on PET, or AVS, or both), two patients with bilateral primary aldosteronism and one indeterminate (due to discordant lateralization on PET and AVS to opposite sides) (Fig. 1). Two patients diagnosed with unilateral primary aldosteronism (both PET-only lateralization) did not proceed with surgery, with one opting for medical therapy, and one deferring surgery for non-medical reasons. Patients who underwent surgery (n = 20) had higher PAC at baseline and post-saline infusion, and higher AVS lateralization ratio, compared with those treated with medications (n = 5) (Table 1).

Discordant lateralization on PET and adrenal vein sampling

One patient had right lateralization on initial AVS, but PET lateralization to a left 2.5 cm adenoma (Fig. 2f). Serum cortisol was unsuppressed (73 nmol/l) after overnight 1 mg dexamethasone suppression. Repeat non-stimulated AVS showed left lateralization (lateralization ratio to left was 2.6, which is greater than the two-fold threshold for non-stimulated AVS to determine lateralization) [8], which was concordant with PET (Supplementary Table S1, https://links.lww.com/HJH/B919). The patient was classified as indeterminate with decision for medical therapy.

F2
FIGURE 2:
11C-Metomidate PET-CT images to illustrate the different permutation of findings between PET-CT and AVS (images from top row are: CT, fused PET-CT, PET, H&E staining, IHC staining with CYP11B2 antibodies of adrenal adenoma. (a) 7 mm right adenoma, initially missed on CT imaging, shows increased uptake on PET with SUVmax of 24.6 (SUV ratio 1.37). AVS lateralized to the right (AVS ratio 22.9). Adenoma contains both ZF and ZG cells on H&E, which have intense CYPB11B2 staining on IHC. (b) 7 mm left adenoma seen on CT shows avid PET uptake, SUVmax 46.7 (SUV ratio 2.05). AVS shows higher levels on the left, but ratio is 2.8 (below 4). Adenoma contains mixture of both ZF and ZG cells, with intense CYP11B2 staining on IHC. (c) 11 mm right adenoma shows SUVmax 39.2, which is less than contralateral gland (SUV ratio 0.65). AVS lateralized to right with ratio of 7.8. Adenoma has predominantly ZF cells, which stain weakly for CYPB11B2. (d) 24 mm right adenoma shows SUVmax 11.6, which is less than contralateral gland (SUV ratio 0.69). AVS showed higher levels on right, but ratio was 3.4 (below 4). Repeated unstimulated AVS showed lateralization ratio 11.8 to right. Adenoma predominantly ZF cells, which have weak and sparse staining for CYP11B2. (e) 15 mm left adenoma does not show increased PET uptake, SUVmax 21.1 (ratio 0.81) consistent with non-functioning adenoma. AVS ratio was 1.3. (f) 15 mm left adenoma shows avid PET uptake with SUVmax 37.2 (SUV ratio 2.48). AVS showed lateralization to opposite right side (AVS ratio 5.0). Repeat unstimulated AVS showed lateralization to left (AVS ratio 2.6). AVS, adrenal vein sampling; H&E, haematoxylin and eosin; IHC, immunohistochemistry; PA, primary aldosteronism; PET-CT, PET-computed tomography; SUVmax, maximal standardized uptake value; ZF, zona fasciculata; ZG, zona glomerulosa.

Adrenalectomy group: PET and adrenal vein sampling results

Individual data regarding subtype tests and treatment are shown in Supplement Table S2, https://links.lww.com/HJH/B919. Twenty patients with unilateral primary aldosteronism underwent surgery: 12 patients had PET+AVS lateralization, four had PET-only lateralization, three had AVS-only lateralization. Amongst the four PET-only lateralization, two patients had AVS lateralization ratios of 2.8 and 3.1, while the other two patients had initial successful but non-lateralizing AVS, and repeat AVS subsequently showed lateralization concordant with PET (repeat AVS done without cosyntropin in one patient, and with cosyntropin in the other). One patient did not fulfil lateralization with either PET or initial AVS (lateralization ratio 3.5), but repeat AVS without cosyntropin showed lateralization [17].

Adrenalectomy group: PASO outcomes

We compared outcomes of patients post-adrenalectomy stratified by their PET and AVS findings: PET+AVS, PET-only or AVS-only (Table 2) [18]. The patient with lateralization only on repeat AVS without cosyntropin was included in the AVS-only group for analysis. Using PASO consensus, all 20 patients (100%) had complete biochemical success (normokalaemia without supplementation and aldosterone-renin ratio less than 550 pmol/l per ng/ml/h). Five patients had complete clinical success, 10 had partial clinical success and five had absent clinical success. Biochemical and clinical outcomes did not differ between the three groups. Four of five patients with absent clinical success had lateralization on both PET and AVS. Amongst 16 cases that PET correctly lateralized, five patients had adenomas less than 8 mm.

TABLE 2 - Comparison of patients who underwent surgery stratified by lateralization on 11C-Metomidate PET-CT and adrenal vein sampling findings
PET+AVS lateralization (n = 12) PET-only lateralization (n = 4) AVS-only lateralizationa (n = 4) P b
Age 50.5 (39.4–58.3) 45.6 (37.0–49.7) 53.8 (42.2–56.8) 0.36
Female / Male 5 / 7 2 / 2 2 / 2 0.94
SUVmax ratio 1.42 (1.32–1.84) 1.78 (1.32–2.03) 0.81 (0.66–1.03) 0·009
AVS lateralization ratio 27.6 (12.3–75.5) 2.9 (1.5–3.3) 8.7 (4.5–19.3) 0.003
PAC, baseline (pmol/l) 748 (1017–1490) 457 (668–978) 645 (953–1687) 0.32
PAC, peripheral vein on day of AVS (pmol/l) 2133 (1094–2978) 2997 (1504–3258) 1681 (770–3354) 0.61
PAC, peripheral vein on day of PET (pmol/l) 485 (312–1199) 623 (471–1753) 609 (249–1316) 0.61
Adenoma diameter <8 mm on histology 3 (25%) 2 (50%) 0 (0%) 0.26
Adenoma diameter (mm) 11 (8–16) 9 (6–14) 11 (9–20) 0.63
Classification by HISTALDO
 Classical 11 4 3 0.48
 Nonclassical 1 0 1
Tumour histology
 ZF predominant adenoma 7 4 3 0.28
 ZG predominant adenoma 5 0 1
Percentage of ZF cells 70 (40–85) 75 (60–90) 85 (35–95) 0.48
Percentage of cells with mild intensity on CYP11B2 85 (80–95) 90 (45–95) 90 (40–90) 0.91
Percentage of cells with maximal intensity on CYP11B2 20 (10–80) 40 (5–70) 15 (5–25) 0.72
H index CYP11B2 105 (90–173) 123 (58–158) 103 (48–116) 0.80
APM present, % 8 (72.7) 3 (75.0) 4 (100.0) 0.51
Biochemical outcomes
 Complete Success 12 (100%) 4 (100%) 4 (100%) 1.0
Clinical outcomes
 Complete success 1 2 2 0.26
 Partial success 7 1 2
 Absent success 4 1 0
Data are presented as n (%), median (IQR).APM, aldosterone-producing micronodules; AVS, adrenal vein sampling; CT, computed tomography; HISTALDO, Histopathology of Primary Aldosteronism; PAC, plasma aldosterone concentration; SUVmax, maximal standardized uptake value; ZF, zona fasciculata; ZG, zona glomerulosa.
aOne patient did not have lateralization on first AVS (stimulated) which was successful, and only on repeat nonstimulated AVS.
bAdjusted by Bonferroni comparison [18].

Adrenalectomy group: HISTALDO (IHC) findings

On histology, 18 out of 20 patients had classical unilateral primary aldosteronism (single aldosterone-producing adenoma, or single aldosterone-producing nodule). One patient with AVS-only lateralization had coexistent aldosterone-producing nodule and aldosterone-producing diffuse hyperplasia. One patient with PET+AVS lateralization had two CYP11B2-negative adenomas, with aldosterone-producing micronodules (APM) noted. Post-surgery, this patient had complete biochemical success and partial clinical success. Fifteen of 20 (75%) patients had APM.

There were no significant differences in CYP11B2 staining or H-score between the groups. However, in patients with the most intense staining for CYP11B2 (empirically ≥40% of tumour area), all seven (100%) had PET lateralization, compared with nine out of 13 (69.2%) with less intense staining, P = 0.10. Figure 2 illustrates the PET findings in the different groups of patients.

Concordance and accuracy of PET and adrenal vein sampling

Overall, PET and AVS conclusions were concordant in 60% (15 of 25). AVS showed lateralization in 16 patients, while PET showed lateralization in 19 patients (14 clear, five likely). For the primary outcome, sensitivity of PET–CT was 80% (16 of 20), 95% CI: 56.3–94.3, and AVS was 75% (15 of 20), 95% CI: 50.9–91.3. For the secondary outcome, the sensitivity and specificity of PET–CT was 81.9% (18 of 22), 95% CI: 59.7–94.8, and 100% (2 of 2), 95% CI: 15.8–100, and AVS was 68.2% (15 of 22), 95% CI: 45.1–86.1 and 100% (2 of 2), 95% CI: 15.8–100, respectively.

The correlation of AVS lateralization ratio with SUVmax ratio was 0.245, P = 0.39 (Fig. 3). Amongst patients with biochemical cure post-surgery, five had AVS lateralization ratio below 4 (false-negative AVS), while four had false-negative PET. Using a stricter SUVmax ratio criterion of 1.25 as previously suggested [16], specificity of PET remained at 100% (2 of 2), but sensitivity decreased to 68.2% (15 of 22) for secondary outcome, and one patient would have missed the opportunity for surgery, as AVS lateralization ratio was 3.4 and SUVmax ratio was 1.22.

F3
FIGURE 3:
Correlation of AVS lateralization ratio with 11C-Metomidate PET-CT SUVmax ratio in all 25 study individuals. AVS lateralization ratio >4 denotes unilateral disease, and 3 -4 is in indeterminate range. SUVmax ratio (SUVmax of tumour to contralateral adrenal gland) >1.2 indicates clear lateralization, while SUVmax ratio (SUVmax of tumor to contralateral adrenal gland) 1 -1.24, or SUVmax ratio (without clear tumour) >1.25 indicates likely lateralization. Filled circles - patients with unilateral PA who underwent surgery, Unfilled circles - patients with unilateral PA who did not undergo surgery, Unfilled squares - patients with bilateral PA, Unfilled triangle - patient with discordant AVS and PET-CT lateralization.

PET as a first-line subtype test

We explored the results of our study if we used the noninvasive PET as a first-line subtype test, and only proceeding with an invasive AVS as second-line test for patients without lateralization on PET (Fig. 4). Eighteen patients (72%) with lateralization on PET could have avoided AVS to diagnose unilateral primary aldosteronism, while one patient with lateralization on PET had indeterminate primary aldosteronism. Of six patients without PET lateralization, three (50%) had correct lateralization after AVS. From our study data, this diagnostic strategy would provide correct diagnosis in 21 patients (84%) with lateralization, and only one patient with unilateral primary aldosteronism would be missed (false-negative rate 4%).

F4
FIGURE 4:
Diagnostic subtype testing using PET as first-line subtype test, followed by AVS as second-line subtype test, based on final clinical diagnosis of patient. AVS, adrenal vein sampling; CT, computed tomography; PET, 11C-Metomidate PET-CT imaging.

DISCUSSION

In our prospective clinical pilot study, the noninvasive 11C-Metomidate PET-CT performed similarly to AVS in subtyping primary aldosteronism, using biochemical cure of primary aldosteronism as reference [10]. All 20 out of 25 patients in the study cohort who underwent surgery achieved biochemical cure, of whom 16 patients were identified on PET, and 15 patients identified on AVS. 11C-Metomidate PET-CT was previously reported to be less sensitive than AVS, but that was using AVS as reference [16]. The PASO consensus has allowed us to compare both subtype tests objectively, and demonstrate cases of unilateral primary aldosteronism, which AVS did not identify.

AVS may fail in nearly 20% of procedures even at highly specialized centres [13]. When cannulation is successful, as it was in all our patients, AVS results may still be affected by cortisol cosecreting APAs, catheter placement or venous anomalies [19]. Asymmetrical aldosterone overproduction may occur in bilateral primary aldosteronism, and possibly explain failure of biochemical cure after unilateral adrenalectomy [20]. Recent data suggest that cosyntropin reduces lateralization index, and hence identifies fewer patients with unilateral primary aldosteronism [21,22]. This was also observed in a few of our patients whereby the initial AVS with cosyntropin showed bilateral primary aldosteronism, but repeat AVS without cosyntropin indicated unilateral primary aldosteronism. It is possible that AVS without cosyntropin may have detected more unilateral primary aldosteronism in our cohort. However, AVS without cosyntropin stimulation is also associated with higher rates of cannulation failure and subsequently, an inconclusive AVS [22]. AVS protocols vary between centres worldwide, regarding use of cosyntropin stimulation, and thresholds to define unilateral primary aldosteronism [8,13]. In our study protocol, all patients underwent AVS with cosyntropin stimulation, similar to a previous randomized controlled trial [9]. Our AVS lateralization ratio threshold is consistent with current guidelines [3,8], and five of our patients cured of primary aldosteronism post-surgery had ratios below this threshold. In addition to cosyntropin use, previous retrospective studies have also demonstrated that patients with lower AVS lateralization ratios may be cured with surgery [22,23]. This has hitherto not been prospectively evaluated, and it is possible that more patients may benefit from surgery.

We diagnosed more patients with unilateral primary aldosteronism using two subtype tests than if only one test was used. Although our study was not adequately powered to demonstrate noninferiority or superiority of PET compared with AVS, we did find that both tests had high specificity in our cohort. All patients achieved complete biochemical success post-surgery, and 75% had partial/complete clinical success. Outcomes did not appear to differ between patients with lateralization on either PET or AVS, although four out of five (80%) of patients with absent clinical success had lateralization on both modalities. Our overall proportion of absent clinical success is comparable with other studies [10,24], and persistence of hypertension post-surgery has been attributed to coexistent essential hypertension or effects of vascular remodelling [25]. Patients classified as absent clinical success may also have BP improvements that do not meet PASO consensus thresholds [26].

AVS and PET were concordant in only 60% of patients, which suggests that they may detect different subtypes of unilateral primary aldosteronism. We assessed all resected adrenals with IHC for CYP11B2 using HISTALDO consensus [27]. All seven tumours with the most intense CYP11B2 expression on IHC showed lateralization on PET, which supports the notion that 11C-metomidate has high affinity for CYP11B2 [16]. As CYP11B2 expression is negatively correlated with tumour size [28], we found PET particularly effective in identifying small functional APA, with five PET-positive adenomas less than 8 mm. Conversely, all four patients with false-negative PET had adenomas more than 1 cm. Three of four patients had suppressed cortisol levels (<50 nmol/l) on the day of PET imaging, while one patient had cortisol level of 137 nmol/l, likely due to a cortisol-cosecreting APA. These PET-negative adenomas showed increased PET uptake, but uptake was lower than the contralateral gland. It is possible that correction of SUVmax for tumour size, or assessment of dynamic PET uptake [29], may help to identify these cases in the future.

Unlike AVS that only provides quantitative information (lateralization ratio), PET provides both quantitative (SUVmax ratio) and qualitative (increased PET uptake within adenoma) information. Hence, we adopted a lower threshold [16] and included adenomas with higher uptake than the contralateral gland. This was supported by our findings of biochemical cure post-surgery in these patients. Using a more stringent SUVmax ratio threshold of 1.25 [16] would have led to one patient missing the opportunity for curative surgery. Patients with single APA (termed ‘classical’ [27]) on IHC assessment have higher rates of biochemical and clinical success, than those with multiple APA or hyperplasia [20,30]. PET may potentially be better at identifying ‘classical’ unilateral primary aldosteronism, as it integrates both structural and functional information. Indeed, one patient with aldosterone-producing hyperplasia had lateralization only on AVS. This will need to be validated in a larger cohort of patients. Another patient had two CYP11B2-negative adrenal nodules despite lateralization on both AVS and PET. Of note, this patient had the lowest SUVmax ratio amongst all PET-positive cases. CYP11B2-negative adrenals have been reported in nearly 10% of patients with primary aldosteronism who have undergone surgery [28,31]. This has been postulated to be due to low CYP11B2 expression, and clinical response does not appear to differ compared with patients with CYP11B2-positive lesions. The histopathology of unilateral primary aldosteronism appears to be heterogeneous with respect to CYP11B2 staining, clinical response as well as underlying somatic mutations, [20,32] and further studies are needed to assess if underlying somatic mutations may explain the differences in lateralization on AVS and PET.

Our findings differ from a recent study by Soinio et al. [33], which did not find 11C-Metomidate PET-CT useful for subtyping primary aldosteronism, with a sensitivity and specificity of 55 and 44%, respectively. Our studies have a few important differences. First, they did not use dexamethasone premedication. As previously shown by Burton et al. [16], which we adopted, dexamethasone suppresses CYP11B1 activity in the normal adrenal tissue, thereby increasing the specificity of metomidate for CYP11B2. Although Soinio et al. [33] studied the effects of dexamethasone premedication in a subset of seven patients, all seven had bilateral primary aldosteronism on AVS. Second, our imaging protocols differ [34] as we followed the protocol by Burton et al. [16]. Third, not all their patients successfully underwent AVS and PET, whereas all our patients successfully completed both tests, allowing direct comparison of the two tests. Fourth, in the previous study, two patients with discordant lateralization on AVS and PET underwent surgery based on AVS lateralization, but biochemical outcome post-surgery was not reported. Our patient with discordant lateralization underwent a second AVS (without cosyntropin), which was concordant with PET. This patient currently still has inadequate BP control on medications, and is considering surgery guided by PET lateralization, which is also important to correct the hypercortisolism (cortisol cosecreting adenoma) [19]. However, dexamethasone has its side effects, with one patient experiencing hyperglycaemia post-dexamethasone, with elevated glucose levels, but resolved subsequently without further intervention.

Our study is the first 11C-Metomidate PET-CT study outside Europe, and first in Asia. Our findings suggest that functional imaging with 11C-Metomidate PET-CT can be a useful alternative to AVS for primary aldosteronism subtyping. As both tests are highly specific and PET is noninvasive, 11C-Metomidate PET-CT could be used as a first-line subtype test, which would have avoided AVS in half of our patients. AVS may then be reserved for patients without PET lateralization, to identify additional patients with unilateral primary aldosteronism. However, 11C has a short half-life of 20 min, which restricts its availability to centres equipped with an onsite cyclotron. Hence, this strategy may not be possible until newer radiotracers (e.g.18F-FAMTO) with a longer half-life are available [35,36]. This will be an important progress to the field, as many centres worldwide also lack the necessary expertise to perform AVS successfully [12,13].

We recognize several limitations in our study. First, our high prevalence of unilateral primary aldosteronism may not be representative of all primary aldosteronism cohorts. This may be due to a referral bias with patients with more severe primary aldosteronism being referred to our centres. Patients with more severe phenotype have higher likelihood of unilateral primary aldosteronism, and may have been more likely to pursue subtyping testing [37]. It could also be due to the use of two subtype tests, as there were patients with unilateral disease identified only on one modality (AVS or PET) that proceeded to surgery. Second, our primary outcome was biochemical cure in patients who underwent surgery, and excluded patients treated medically. As it is not ethical to offer surgery to all patients, we cannot ascertain false-negative test results (underlying unilateral primary aldosteronism in patients classified as bilateral primary aldosteronism). However, a high proportion of our patients underwent adrenalectomy, which allowed comparison of PET and AVS to post-surgery biochemical cure (primary outcome). Third, the decision for surgery was made at the multidisciplinary meeting based on both PET and AVS results as per our study protocol. Importantly, PET was reported independently by physicians blinded to AVS findings. This approach was chosen as PET is a relatively new test, and reflects clinical practice when all tests are used to guide clinical decision. Fourth, we performed repeat AVS in several patients despite a prior successful AVS, which was not predefined in our protocol. However, we used the first successful AVS results for analysis. As mentioned [37] earlier, use of cosyntropin may affect AVS interpretation, and repeat AVS in several of our cases reaffirmed the accuracy of PET. Finally, although we standardized measurements of clinic BP, 24-h ambulatory BP may be a better measure of BP control. However, biochemical cure is the best determinant of unilateral primary aldosteronism and we were able to fully assess all patients for clinical and biochemical outcomes using PASO consensus, and histological outcomes using HISTALDO consensus.

In conclusion, our pilot study found that 11C-Metomidate PET-CT performed similarly to AVS for subtyping of primary aldosteronism, and our findings need to be further validated in a larger study. 11C-Metomidate PET-CT identified additional cases of unilateral primary aldosteronism, which were not identified with AVS. 11C-Metomidate PET-CT may be a noninvasive alternative to AVS, and use of both subtype tests can potentially identify more patients with unilateral surgically curable primary aldosteronism.

This is the first prospective clinical trial demonstrating that 11C-Metomidate PET-CT (with dexamethasone pre-treatment) can accurately subtype patients with primary aldosteronism, and offer a noninvasive alternative to AVS. 11C-Metomidate PET-CT also identified cases of unilateral primary aldosteronism that AVS failed to identify, which were all confirmed with biochemical cure after unilateral adrenalectomy. Only half of the patients with unilateral primary aldosteronism had lateralization of both AVS and PET, which suggests that they may detect different subtypes of unilateral primary aldosteronism. Hence, 11C-Metomidate PET-CT may be a noninvasive subtype test for primary aldosteronism, potentially allowing some patients to avoid AVS. In addition, use of both subtype tests together may help detect more patients with unilateral surgically curable primary aldosteronism.

ACKNOWLEDGEMENTS

The authors thank Professor Morris Brown (William Harvey Research Institute, UK) for his valuable assistance and support in development of 11C-Metomidate PET-CT in Singapore, Professor Mark Gurnell, Dr Heok Cheow and Dr Russell Senanayake (Addenbrookes Hospital, University of Cambridge, UK) for their guidance with interpretation of 11C-Metomidate PET-CT images and valuable comments to the manuscript, Ms Dahlia Ho (Histopathology, CGH) for her technical support with immunohistochemistry staining.

This study was funded by National Medical Research Council (NMRC), Singapore. R.S. Foo was supported by NMRC, Singapore and A∗Star Biomedical Research Council. T.H. Puar was supported by the NMRC Research Training Fellowship award, Singapore.

The data that support the findings of this study are available from the corresponding author upon request.

Clinical Trials Registration number: Clinicaltrials.gov NCT03990701.

An abstract of this manuscript was accepted for oral presentation at ENDO 2020.

Conflicts of interest

The authors have nothing to declare.

Graphical abstract: https://links.lww.com/HJH/B920.

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Complete list of investigators provided in the data supplement, https://links.lww.com/HJH/B919.

Keywords:

adrenalectomy; adrenocortical adenoma; diagnostic testing; functional imaging; hyperaldosteronism; subtyping

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