The effect of P-glycoprotein function inhibition with cyclosporine A on the biodistribution of Tc-99m sestamibi

The effects of pharmacological interventions, exercise, and dietary supplements on extra-cardiac radioactivity in myocardial perfusion single-photon emission computed tomography imaging

Myocardial perfusion imaging (MPI) as an imaging modality plays a key role in the monitoring of patients with cardiovascular disease. MPI enables the assessment of cardiovascular disease, the effectiveness of therapy, and viable myocardial tissue. However, MPI suffers from some downfalls and limitations, which can influence its clinical applications. These limitations can arise from the patient's condition, equipment, or the actions of the technologist. In this review, we mainly focused on the different effective parameters on radioactivity uptake of organs including liver, intestines, stomach, and gall bladder and how they affect the quality of the acquired images in nuclear medicine. More importantly, we cover how different suggested medicines, foods and exercise alleviative this problem.

Old tracer for a new purpose: potential role for 99mTc-2-Methoxyisobutylisonitrile (99mTc-MIBI) in renal transplant care

AIM

Calcineurin inhibitors are substrates for P-glycoprotein (P-gp), the expression of which is associated with ABCB1 C3435T polymorphism. Individual P-gp response to calcineurin inhibitor may be linked to nephrotoxicity or rejection. Tc-2-Methoxyisobutylisonitrile (Tc-MIBI) is also a P-gp substrate. The aim of this study, therefore, was to determine Tc-MIBI organ kinetics and compare them with ABCB1 genotype with a view to replacing Tc-mercaptoacetyltriglycine (Tc-MAG3) with Tc-MIBI in renal transplant care.

METHODS

Thirty prospective donors (13 male) were imaged for 20 min after administration of Tc-MIBI (400 MBq) intravenously. Posterior images of the abdomen were acquired at 30 and 120 min. Organ 30 min/peak count rate ratios and exponential two-point (30-120 min) rate constants (k, min) were calculated. Nineteen donors were genotyped for C3435T (exon 26), G2677T (exon 21), C1236T (exon 12), and G1199A (exon 11) ABCB1 polymorphisms using a PCR-based technique.

RESULTS

Tc-MIBI and Tc-MAG3 gave similar perfusion images. Although their patterns of renal elimination were different, differential renal function was not significantly different. There was a negative trend between the hepatic 30 min/peak ratio and C3435T genotype (CC: 0.8374 ± 0.0502; TC: 0.6806 ± 0.1300; TT: 0.6919 ± 0.1506; P=0.083). Renal k showed a negative trend with C3435T (CC: 0.0021 ± 0.0020; TC: 0.0037 ± 0.0013; TT: 0.0040 ± 0.0012 min; P=0.087) but with no other genotypes. There were no significant sex-related differences in Tc-MIBI kinetics.

CONCLUSION

Tc-MIBI can replace Tc-MAG3 for pretransplant workup. The ABCB1 C3435T polymorphism may influence Tc-MIBI kinetics and thus have a role in renal transplant care. Further prospective trials are required to establish the full potential of Tc-MIBI in renal transplant management.

Imaging of multidrug resistance in cancer

Primary intrinsic and/or acquired multidrug resistance (MDR) is the main obstacle to successful cancer treatment. Functional molecular imaging of MDR in cancer using single photon or positron emitters may be helpful to identify multidrug-resistant tumours and predict not only those patients who are resistant to treatment, with a clinically unfavourable prognosis, but also those who are susceptible to the development of drug toxicity or even certain tumours . Variations in the mdr1 gene product may directly affect the therapeutic effectiveness, and single nucleotide polymorphisms for the mdr1 gene may be associated with altered oral bioavailability of MDR1 substrates, drug resistance, and a susceptibility to some human diseases. The challenge of translating the concept of MDR modulation in vivo involves a complex cellular interplay between both malignant and normal cells. Integration and correlation of functional single photon emission tomography or positron emission tomography imaging findings with mdr1 genotype and clinical data may contribute to efficient management by selecting cancer patients with the appropriate molecular phenotype for maximal individual therapeutic benefit, as well as those who are non-responders. This review describes a role for functional imaging of classical mechanisms of MDR with an emphasis on readily available [^99mTc]MIBI scintigraphy. MIBI scintigraphy has been shown to be a non-invasive cost-effective in vivo assay of ATP-binding cassette transporters associated with MDR in cancer, including P-glycoprotein, multidrug-resistant protein 1 and breast cancer resistant protein. New imaging agents for molecular targets such as vascular endothelial growth factor and HER2 receptors, may potentially be combined with MDR imaging substrates to more accurately predict the therapeutic response to anticancer drugs, guiding individualised treatment while minimising the economic health costs of ineffective therapy in an era of personalised medicine.

Subcellular distribution and metabolism studies of the potential myocardial imaging agent [99mTc(N)(DBODC)(PNP5)]+

99mTc(N)-DBODC5 is the lead compound of a new series of monocationic 99mTc(N)-based potential myocardial imaging agents that exhibit original biodistribution properties. This study was addressed to elucidate the mechanisms of distribution, retention, and elimination of this promising 99mTc(N)-agent.

METHODS

The sex-related in vitro and in vivo stability and the subcellular distribution of 99mTc(N)-DBODC5 were investigated. Studies were performed by considering binding to the serum proteins; stability in rat serum, human serum, and rat liver homogenates; and the chemical integrity of the complex after extraction from rat tissues such as heart, liver, and kidney, as well as from intestinal fluids and urine. The effect of cyclosporin A on the in vivo pharmacokinetic properties of 99mTc(N)-DBODC5 was also evaluated. Subcellular distribution of 99mTc(N)-DBODC5 in ex vivo rat heart was determined by standard differential centrifugation techniques.

RESULTS

No significant in vitro serum protein binding and no notable biotransformation of the native compound into different species by the in vitro action of the serum and liver enzymes was evidenced. In vivo experiments showed that sex affects the pharmacokinetic profile of the 99mTc(N)-complexes including metabolism and excretion. Chromatographic profiles of 99mTc(N)-radioactivity extracted from tissues and fluids of female rats were always coincident with the control. Conversely, a small percentage of metabolized species was detected by high-performance liquid chromatography in liver extracts of male rats. Furthermore, administration of cyclosporin A caused a significant reduction of lung, liver, and kidney washout along with a considerable variation in activity distribution in the intestinal tract in both male and female rats, thus indicating a possible implication of Pgp transporters in determining the biologic behavior of 99mTc(N)-DBODC5. However, this phenomenon was more pronounced in females. Subcellular distribution studies showed that 86.3% +/- 7.4% of 99mTc(N)-DBODC5 was localized into mitochondrial fraction as a result of the interaction with the negative membrane potential.

CONCLUSION

Evidence showing that the new 99mTc(N)-myocardial tracers behave as multidrug resistance-associated protein P-glycoprotein substrates, combined with their selective mitochondrial accumulation, strongly supports the possibility that diagnostic application of 99mTc(N)-DBODC5 can be extended to tumor imaging and noninvasive multidrug resistance studies.

P-glycoprotein expression is associated with sestamibi washout in primary hyperparathyroidism

Background

The detection of parathyroid adenomas by 99mTc-labelled hexakis 2-methoxyisobutyl isonitrile (sestamibi) scintigraphy is influenced by several factors, including tumour size and serum level of parathyroid hormone (PTH). This study examined the relationship between sestamibi accumulation and multidrug resistance (MDR)-related P-glycoprotein (P-gp) expression in a large series of surgically excised parathyroid tumours.

Methods

Seventy-eight patients underwent dual-phase sestamibi imaging before parathyroidectomy. Expression of P-gp within tumour cells was assessed by immunohistochemistry. Tumour size was measured and the ellipsoid volume calculated. Scan results were analysed in relation to preoperative serum levels of calcium and PTH, P-gp expression and tumour volume.

Results

Sixty-four of the 78 sestamibi scans were positive and 14 negative. Smaller adenomas (less than 0·5 cm3) were more likely to be sestamibi negative than larger lesions (P = 0·006). Ten of 14 adenomas with negative imaging showed strong P-gp membrane positivity and 45 of 64 lesions with a positive scan did not show P-gp membrane expression, indicating a significant association between high P-gp membrane immunoreactivity and negative sestamibi result (P = 0·006).

Conclusion

These data suggest an association between P-gp membrane expression and false-negative sestamibi scan result. Inhibition of the P-gp transmembrane pump using MDR modulators may therefore improve the sensitivity of sestamibi scintigraphy.