Metabolic and pharmacokinetic considerations in the design of 2-phenyl substituted metyrapone derivatives: 2-methoxyphenylmetyrapone as a radioligand for functional diagnosis of adrenal pathology

Radiopharmaceuticals for Treatment of Adrenocortical Carcinoma

Adrenocortical carcinoma (ACC) represents a rare tumor entity with limited treatment options and usually rapid tumor progression in case of metastatic disease. As further treatment options are needed and ACC metastases are sensitive to external beam radiation, novel theranostic approaches could complement established therapeutic concepts. Recent developments focus on targeting adrenal cortex-specific enzymes like the theranostic twin [123/131I]IMAZA that shows a good image quality and a promising therapeutic effect in selected patients. But other established molecular targets in nuclear medicine such as the C-X-C motif chemokine receptor 4 (CXCR4) could possibly enhance the therapeutic regimen as well in a subgroup of patients. The aims of this review are to give an overview of innovative radiopharmaceuticals for the treatment of ACC and to present the different molecular targets, as well as to show future perspectives for further developments since a radiopharmaceutical with a broad application range is still warranted.

Adrenal functional imaging - which marker for which indication?

PURPOSE OF REVIEW

In recent years, a broad spectrum of molecular image biomarkers for assessment of adrenal functional imaging have penetrated the clinical arena. Those include positron emission tomography and single photon emission computed tomography radiotracers, which either target glucose transporter, CYP11B enzymes, C-X-C motif chemokine receptor 4, norepinephrine transporter or somatostatin receptors. We will provide an overview of key radiopharmaceuticals and determine their most relevant clinical applications, thereby providing a roadmap for the right image biomarker at the right time for the right patient.

RECENT FINDINGS

Numerous radiotracers for assessment of adrenal incidentalomas ([18F]FDG; [123I]IMTO/IMAZA), ACC ([123I]IMTO/IMAZA; [18F]FDG; [68Ga]PentixaFor), pheochromocytomas and paragangliomas ([123I]mIBG; [18F]flubrobenguane; [18F]AF78; [68Ga]DOTATOC/-TATE), or primary aldosteronism ([11C]MTO, [68Ga]PentixaFor) are currently available and have been extensively investigated in recent years. In addition, the field is currently evolving from adrenal functional imaging to a patient-centered adrenal theranostics approach, as some of those radiotracers can also be labeled with ß-emitters for therapeutic purposes.

SUMMARY

The herein reviewed functional image biomarkers may not only allow to increase diagnostic accuracy for adrenal gland diseases but may also enable for achieving substantial antitumor effects in patients with adrenocortical carcinoma, pheochromocytoma or paraganglioma.

Targeted Molecular Imaging in Adrenal Disease-An Emerging Role for Metomidate PET-CT

Adrenal lesions present a significant diagnostic burden for both radiologists and endocrinologists, especially with the increasing number of adrenal ‘incidentalomas’ detected on modern computed tomography (CT) or magnetic resonance imaging (MRI). A key objective is the reliable distinction of benign disease from either primary adrenal malignancy (e.g., adrenocortical carcinoma or malignant forms of pheochromocytoma/paraganglioma (PPGL)) or metastases (e.g., bronchial, renal). Benign lesions may still be associated with adverse sequelae through autonomous hormone hypersecretion (e.g., primary aldosteronism, Cushing’s syndrome, phaeochromocytoma). Here, identifying a causative lesion, or lateralising the disease to a single adrenal gland, is key to effective management, as unilateral adrenalectomy may offer the potential for curing conditions that are typically associated with significant excess morbidity and mortality. This review considers the evolving role of positron emission tomography (PET) imaging in addressing the limitations of traditional cross-sectional imaging and adjunctive techniques, such as venous sampling, in the management of adrenal disorders. We review the development of targeted molecular imaging to the adrenocortical enzymes CYP11B1 and CYP11B2 with different radiolabeled metomidate compounds. Particular consideration is given to iodo-metomidate PET tracers for the diagnosis and management of adrenocortical carcinoma, and the increasingly recognized utility of ¹¹C-metomidate PET-CT in primary aldosteronism.

Comparison of three different purification methods for the routine preparation of [11C] Metomidate

PET with (R)-[O-methyl-11C] metomidate ([11C] MTO) is an attractive method for the characterisation of adrenal masses discriminating lesions of adrenal cortical origin from noncortical lesions. [11C] MTO was prepared by the reaction of [11C] methyliodide with the corresponding free acid. Three purification methods have been compared. The method of choice uses preparative HPLC with a ready-to-use weak acidic solvent.

Pharmacokinetics of 2-methoxyphenylmetyrapone and 2-bromophenylmetyrapone in rats

The pharmacokinetics of two 2-substituted phenylmetyrapone analogues, 2-methoxyphenylmetyrapone (2-MPMP) and 2-bromophenylmetyrapone (2-BrPMP), developed as potential adrenal imaging agents, were investigated in conscious male rats following an intravenous dose of 25 mg/kg. Arterial blood samples (0.25 ml) were collected at various intervals for up to 7 h after dose and subjected to reversed-phase HPLC analysis. Blood concentrations versus time profile for each compound was determined and the pharmacokinetic parameters calculated using the model-independent approach. Blood concentrations of 2-MPMP declined biexponentially with mean initial (t1/2alpha) and terminal (t1/2beta) half-lives of 3.6 and 23.1 min, respectively. The corresponding area under the curve (AUC(0-infinity)) was 159.3 microg x min/ml, the total blood clearance (CI) was 158.3 ml/min and the volume of distribution (Vd) was 5.2 l. Two metabolites of 2-MPMP, namely 2-hydroxyphenylmetyrapone (2-OHPMP) and 2-methoxyphenylmetyrapone N-oxide (2-MPMP-NO), were detected in the blood and their elimination from blood was almost parallel to that of the parent compound. The maximum blood concentrations (Cmax) of 2-OHPMP and 2-MPMP-NO were approximately 0.9 and 1.7 microg/ml, respectively. Blood concentrations of 2-BrPMP declined monoexponentially with a mean t1/2beta of 12.0 min. The pharmacokinetic parameters for 2-BrPMP were: AUC(0-infinity), 193.7 microg x min/ml; Cl, 131.7 ml/min and Vd, 2.3 l. 2-Bromophenylmetyrapone N-oxide was the only one metabolite detected in the blood, its Cmax and AUC0-infinity were 10.1 microg/ml and 1690.0 microg x min/ml, respectively.

Simultaneous analysis of 2-methoxyphenylmetyrapone and its seven potential metabolites by high-performance liquid chromatography

A sensitive and specific high-performance liquid chromatographic (HPLC) assay has been developed for the quantification of 2-methoxyphenylmetyrapone (2-MPMP) and its seven potential metabolites in rat urine and whole blood. 2-MPMP, 2-hydroxyphenylmetyrapone and their N-oxides, together with 2-methoxyphenylmetyrapol, 2-hydroxyphenylmetyrapol and their N-oxides were separated on an Isco Spherisorb ODS-2 reversed-phase column (250 x 4.6 mm, I.D., 5 microm), with an Isco Spherisorb ODS-2 guard cartridge (10 x 4.6 mm I.D.). A gradient elution was employed using solvent system A (acetonitrile-water-triethylamine-acetic acid, 27.3:69.1:0.9:2.7%, v/v) and solvent system B (methanol), the gradient program being as follows: initial 0-4 min A:B=74:26; 4-10 min linear change to A:B=50:50; 10-16 min maintain A:B=50:50; 16 min return to initial conditions (A:B=74:26). Flow-rate was maintained at 1.25 ml/min, and the eluent monitored using a diode array multiple wavelength UV detector set at 260 nm. Most of the analytes were baseline resolved, and analysis of samples recovered from blood or urine (pH 12, 3 x 5 ml of dichloromethane, recovery approximately 20-95%) revealed no interference from any co-extracted endogenous compounds in the biological matrices, except for 2-hydroxyphenylmetyrapol N-oxide (2-OHPMPOL-NO) at low concentrations. The calibrations (n=6) were linear (r > or = 0.996) for all analytes (approximately 0.5-100 microg/ml), with acceptable inter- and intra-day variability. Subsequent validation of the assay revealed acceptable precision, as measured by coefficient of variation (C.V.) at the low (0.5 mg/ml), medium (50 microg/ml) and high (100 microg/ml) concentrations. The limits of detection for 2-MPMP and their available potential metabolites, except 2-OHPMPOL-NO, in rat urine and blood were both 0.5 microg/ml, respectively.