Prediction of clinically relevant hyperkalemia in patients treated with peptide receptor radionuclide therapy

90Y/177Lu-DOTATOC: From Preclinical Studies to Application in Humans

The PRRT (Peptide Receptor Radionuclide Therapy) is a promising modality treatment for patients with inoperable or metastatic neuroendocrine tumors (NETs). Progression-free survival (PFS) and overall survival (OS) of these patients are favorably comparable with standard therapies. The protagonist in this type of therapy is a somatostatin-modified peptide fragment ([Tyr3] octreotide), equipped with a specific chelating system (DOTA) capable of creating a stable bond with β-emitting radionuclides, such as yttrium-90 and lutetium-177. In this review, covering twenty five years of literature, we describe the characteristics and performances of the two most used therapeutic radiopharmaceuticals for the NETs radio-treatment: [⁹⁰Y]Y-DOTATOC and [¹⁷⁷Lu]Lu-DOTATOC taking this opportunity to retrace the most significant results that have determined their success, promoting them from preclinical studies to application in humans.

Improving the Theranostic Potential of Exendin 4 by Reducing the Renal Radioactivity through Brush Border Membrane Enzyme-Mediated Degradation

As highly expressed in insulinomas, the glucagon-like peptide-1 receptor (GLP-1R) is believed to be an attractive target for diagnosis, localization, and treatment with radiolabeled exendin 4. However, the high and persistent radioactivity accumulation of exendin 4 in the kidneys limits accurate diagnosis and safe, as well as effective, radiotherapy in insulinomas. In this study, we intend to reduce the renal accumulation of radiolabeled exendin 4 through degradation mediated by brush border membrane enzymes. A new exendin 4 ligand NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 containing Met-Val-Lys (MVK) linker between the peptide and 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) chelator was synthesized and labeled with ⁶⁸Ga. The in vitro mouse serum stability and cell binding affinity of the tracer were evaluated. Initial in vitro cleavage of the linker was determined by incubation of a model compound Boc-MVK-Dde with brush border membrane vesicles (BBMVs) with and without the inhibitor of neutral endopeptidase (NEP). Further cleavage studies were performed with the full structure of NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4. Kidney and urine samples were collected in the in vivo metabolism study after intravenous injection of ⁶⁸Ga-NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4. The microPET images were acquired in INS-1 tumor model at different time points; the radioactivity uptake of ⁶⁸Ga-NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 in tumor and kidneys were determined and compared with the control radiotracer without MVK linker. ⁶⁸Ga-NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 was stable in mouse serum. The MVK modification did not affect the affinity of NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 toward GLP-1R. The in vitro cleavage study and in vivo metabolism study confirmed that the MVK sequence can be recognized by BBM enzymes and cleaved at the amide bond between Met and Val, thus releasing the small fragment containing Met. MicroPET images showed that the tumor uptake of ⁶⁸Ga-NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 was comparable to that of the control, while the kidney uptake was significantly reduced. As a result, more favorable tumor to kidney ratios were achieved. In this study, a novel exendin 4 analogue, NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4, was successfully synthesized and labeled with ⁶⁸Ga. With the cleavable MVK sequence, this ligand could be cleaved by the enzymes on kidneys, and releasing the fragment of ⁶⁸Ga-NOTA-Met-OH, which will rapidly excrete from urine. As the high and consistent renal radioactivity accumulation could be significantly reduced, NOTA-MVK-Cys⁴⁰-Leu¹⁴-Exendin 4 shows great potential in the diagnosis and radiotherapy for insulinoma.

Hyperkalemia in patients treated with endoradiotherapy combined with amino acid infusion is associated with severe metabolic acidosis

Background

Amino acid co-infusion for renal protection in endoradiotherapy (ERT) applied as prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) or peptide receptor radionuclide therapy (PRRT) has been shown to cause severe hyperkalemia. The pathophysiology behind the rapid development of hyperkalemia is not well understood. We hypothesized that the hyperkalemia should be associated with metabolic acidosis.

Results

Twenty-two patients underwent ERT. Prior to the first cycle, excretory kidney function was assessed by mercapto-acetyltriglycine (MAG-3) renal scintigraphy, serum biochemistry, and calculated glomerular filtration rate (eGFR). All patients received co-infusion of the cationic amino acids L-arginine and L-lysine for nephroprotection. Clinical symptoms, electrolytes, and acid-base status were evaluated at baseline and after 4 h.

No patient developed any clinically relevant side effects. At baseline, acid base status and electrolytes were normal in all patients. Excretory kidney function was normal or only mildly impaired in all except two patients with stage 3 renal insufficiency. All patients developed hyperkalemia. Base excess and HCO₃⁻ were significantly lower after 4 h. In parallel, mean pH dropped from 7.36 to 7.29. There was a weak association between calculated (r = − 0.21) as well as MAG-3-derived GFR (r = − 0.32) and the rise in potassium after 4 h.

Conclusion

Amino acid co-infusion during ERT leads to severe metabolic acidosis which induces hyperkalemia by potassium hydrogen exchange. This novel finding implies that commercially available bicarbonate solutions might be an easy therapeutic option to correct metabolic acidosis rapidly.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0370-z) contains supplementary material, which is available to authorized users.

The impact of 177Lu-octreotide therapy on 99mTc-MAG3 clearance is not predictive for late nephropathy

Peptide Receptor Radionuclide Therapy (PRRT) for the treatment of neuroendocrine tumors may lead to kidney deterioration. This study aimed to evaluate the suitability of ^99mTc-mercaptoacetyltriglycine (^99mTc­-MAG3) clearance for the early detection of PRRT-induced changes on tubular extraction (TE). TE rate (TER) was measured prior to 128 PRRT cycles (7.6±0.4 GBq ¹⁷⁷Lu-octreotate/octreotide each) in 32 patients. TER reduction during PRRT was corrected for age-related decrease and analyzed for the potential to predict loss of glomerular filtration (GF). The GF rate (GFR) as measure for renal function was derived from serum creatinine. The mean TER was 234 ± 53 ml/min/1.73 m² before PRRT (baseline) and 221 ± 45 ml/min/1.73 m² after a median follow-up of 370 days. The age-corrected decrease (mean: −3%, range: −27% to +19%) did not reach significance (p=0.09) but significantly correlated with the baseline TER (Spearman p=−0.62, p<0.001). Patients with low baseline TER showed an improved TER after PRRT, high decreases were only observed in individuals with high baseline TER. Pre-therapeutic TER data were inferior to plasma creatinine-derived GFR estimates in predicting late nephropathy. TER assessed by ^99mTc-MAG3­clearance prior to and during PRRT is not suitable as early predictor of renal injury and an increased risk for late nephropathy.

Apparent pacemaker dysfunction during peptide receptor radionuclide therapy for neuroendocrine tumor

This case is a reminder not to overlook rare causes of electrolyte shifts, which may cause reversible changes in pacemaker pacing thresholds.

[177Lu-DOTA0,Tyr3]-octreotate in the treatment of midgut neuroendocrine tumors

Midgut neuroendocrine tumors (NETs) are relatively rare and remarkably heterogeneous. Although recent developments for pancreatic NETs have brought multiple new therapies to patients who need them, there has been little observed efficacy against midgut NETs. Peptide receptor radionuclide therapy utilizes somatostatin analogs conjugated to radioactive isotopes in order to deliver high doses of radiation directly to tumor cells, which express somatostatin receptors. Peptide receptor radionuclide therapy with [(177)Lu-DOTA(0),Tyr(3)]-octreotate (DOTATATE) has been reported and investigated for more than a decade, and the randomized controlled NETTER-1 study of this agent has recently been reported to show promising results. In this article, we will summarize and evaluate the rationale and existing clinical data for the activity of DOTATATE in midgut NETs, to give context for the interpretation of NETTER-1 results when they are fully available.

Human Anti-Oxidation Protein A1M--A Potential Kidney Protection Agent in Peptide Receptor Radionuclide Therapy

Peptide receptor radionuclide therapy (PRRT) has been in clinical use for 15 years to treat metastatic neuroendocrine tumors. PRRT is limited by reabsorption and retention of the administered radiolabeled somatostatin analogues in the proximal tubule. Consequently, it is essential to develop and employ methods to protect the kidneys during PRRT. Today, infusion of positively charged amino acids is the standard method of kidney protection. Other methods, such as administration of amifostine, are still under evaluation and show promising results. α₁-microglobulin (A1M) is a reductase and radical scavenging protein ubiquitously present in plasma and extravascular tissue. Human A1M has antioxidation properties and has been shown to prevent radiation-induced in vitro cell damage and protect non-irradiated surrounding cells. It has recently been shown in mice that exogenously infused A1M and the somatostatin analogue octreotide are co-localized in proximal tubules of the kidney after intravenous infusion. In this review we describe the current situation of kidney protection during PRRT, discuss the necessity and implications of more precise dosimetry and present A1M as a new, potential candidate for renal protection during PRRT and related targeted radionuclide therapies.