Detection of prostate cancer with 18F-DCFPyL PET/CT compared to final histopathology of radical prostatectomy specimens: is PSMA-targeted biopsy feasible? The DeTeCT trial.

PURPOSE: In primary prostate cancer (PCa) patients, accurate staging and histologic grading are crucial to guide treatment decisions. 18F-DCFPyL (PSMA)-PET/CT has been successfully introduced for (re)staging PCa, showing high accuracy to localise PCa in lymph nodes and/or osseous structures. The diagnostic performance of 18F-DCFPyL-PET/CT in localizing primary PCa within the prostate gland was assessed, allowing for PSMA-guided targeted-prostate biopsy. METHODS: Thirty patients with intermediate-/high-risk primary PCa were prospectively enrolled between May 2018 and May 2019 and underwent 18F-DCFPyL-PET/CT prior to robot-assisted radical prostatectomy (RARP). Two experienced and blinded nuclear medicine physicians assessed tumour localisation within the prostate gland on PET/CT, using a 12-segment mapping model of the prostate. The same model was used by a uro-pathologist for the RARP specimens. Based on PET/CT imaging, a potential biopsy recommendation was given per patient, based on the size and PET-intensity of the suspected PCa localisations. The biopsy recommendation was correlated to final histopathology in the RARP specimen. Sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) for clinically significant PCa (csPCa, Gleason score ≥ 3 + 4 = 7) were assessed. RESULTS: The segments recommended for potential targeted biopsy harboured csPCA in 28/30 patients (93%), and covered the highest Gleason score PCa segment in 26/30 patient (87%). Overall, 122 of 420 segments (29.0%) contained csPCa at final histopathological examination. Sensitivity, specificity, PPV and NPV for csPCa per segment using 18F-DCFPyL-PET/CT were 61.4%, 88.3%, 68.1% and 84.8%, respectively. CONCLUSIONS: When comparing the PCa-localisation on 18F-DCFPyL-PET/CT with the RARP specimens, an accurate per-patient detection (93%) and localisation of csPCa was found. Thus, 18F-DCFPyL-PET/CT potentially allows for accurate PSMA-targeted biopsy.

A prospective, multicenter head-to-head comparative study in patients with primary high-risk prostate cancer investigating the bone lesion detection of conventional imaging and 18F-PSMA-PET/CT.

PURPOSE: Prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) is an emerging staging tool for patients with primary high-risk prostate cancer (PCa). Patients with primary metastatic disease are staged using PSMA-PET/CT imaging, while previously published randomized clinical trials relied on conventional imaging (i.e., bone scintigraphy (BS) results. The aim of this study was to compare the ability of bone metastatic lesion detection and changes in staging for 18F-PSMA-PET/CT versus BS in high-risk PCa patients. METHODS: 79 patients with high-risk PCa were prospectively staged using BS and subsequent 18F-PSMA-PET/CT before initial therapy. Patients who presented with a BS showing no metastases represented Group 1, and patients with a BS showing low-volume disease according to the CHAARTED criteria (<4 bone metastases, no metastases outside vertebral column or pelvis and no visceral metastases) represented Group 2. Metastatic risk group according to CHAARTED and treatment strategies based on both imaging modalities were assessed. RESULTS: A change of CHAARTED risk group was observed in 9/70 (12.8%) of patients in Group 1. In Group 2, a change of risk group was found in 66.7% of patients, due to either upstaging (4/9 patients (44.4%)) and downstaging (2/9 patients (22.2%)). Treatment changes due to use of a different imaging modality occurred in almost 20% of patients. CONCLUSION: In patients with negative for cancer results on BS, upstaging on 18F-PSMA-PET/CT occurred only infrequently. Moreover, 18F-PSMA-PET/CT resulted in both upstaging and downstaging in a substantial subset of patients with low-volume metastatic disease on BS. Treatment changes occurred in almost 20% of cases depending on imaging results.

Physiologically Based Pharmacokinetic (PBPK) Modeling to Predict PET Image Quality of Three Generations EGFR TKI in Advanced-Stage NSCLC Patients.

INTRODUCTION: Epidermal growth factor receptor (EGFR) mutated NSCLC is best treated using an EGFR tyrosine kinase inhibitor (TKI). The presence and accessibility of EGFR overexpression and mutation in NSCLC can be determined using radiolabeled EGFR TKI PET/CT. However, recent research has shown a significant difference between image qualities (i.e., tumor-to-lung contrast) in three generation EGFR TKIs: 11C-erlotinib, 18F-afatinib and 11C-osimertinib. In this research we aim to develop a physiological pharmacokinetic (PBPK)-model to predict tumor-to-lung contrast and as a secondary outcome the uptake of healthy tissue of the three tracers. METHODS: Relevant physicochemical and drug specific properties (e.g., pKa, lipophilicity, target binding) for each TKI were collected and applied in established base PBPK models. Key hallmarks of NSCLC include: immune tumor deprivation, unaltered tumor perfusion and an acidic tumor environment. Model accuracy was demonstrated by calculating the prediction error (PE) between predicted tissue-to-blood ratios (TBR) and measured PET-image-derived TBR. Sensitivity analysis was performed by excluding each key component and comparing the PE with the final mechanistical PBPK model predictions. RESULTS: The developed PBPK models were able to predict tumor-to-lung contrast for all EGFR-TKIs within threefold of observed PET image ratios (PE tumor-to-lung ratio of -90%, +44% and -6.3% for erlotinib, afatinib and osimertinib, respectively). Furthermore, the models depicted agreeable whole-body distribution, showing high tissue distribution for osimertinib and afatinib and low tissue distribution at high blood concentrations for erlotinib (mean PE, of -10.5%, range -158%-+190%, for all tissues). CONCLUSION: The developed PBPK models adequately predicted the image quality of afatinib and osimertinib and erlotinib. Some deviations in predicted whole-body TBR lead to new hypotheses, such as increased affinity for mutated EGFR and active influx transport (erlotinib into excreting tissues) or active efflux (afatinib from brain), which is currently unaccounted for. In the future, PBPK models may be used to predict the image quality of new tracers.

Determining the diagnostic value of PSMA-PET/CT imaging in patients with persistent high prostate specific antigen levels and negative prostate biopsies.

PURPOSE: To assess the diagnostic performance of prostate specific membranous antigen (PSMA) positron emission tomography/computed tomography (PET/CT) imaging to localize primary prostate cancer (PCa) in men with persistent elevated prostate-specific antigen (PSA) levels and previous prostate biopsies that were negative for PCa. METHODS: In this study, 34 men with persistently elevated PSA-levels, previous negative for PCa biopsies and who subsequently underwent diagnostic PSMA-PET/CT imaging were retrospectively evaluated. Men were divided into 3 groups: 1. 12 men with a previous negative mpMRI scan (PI-RADS 1-2) 2. 17 men with a positive mpMRI scan (PI-RADS 3-5), but negative MRI-targeted biopsies and 3. Four men in whom mpMRI was contraindicated. If PSMA-avid lesions were seen, patients underwent 2-4 cognitive targeted biopsies in combination with systematic biopsies. The detection rate of PSMA-PET/CT for PCa, and the accuracy of (possible) targeted biopsies were calculated. RESULTS: Included men had a median PSA-level of 22.8 ng/mL (Interquartile Range 15.6-30.0) at the time of PSMA-PET/CT. Elevated PSMA-ligand uptake in the prostate suspicious for PCa was observed in 22/34 patients (64.7%). In 18/22 patients (54.5%), PSMA-targeted prostate biopsies were performed. In 3/18 patients (16.6%), the targeted biopsies showed International Society of Urological Pathology (ISUP) score 1-2 PCa. The other men had inflammation or benign findings after histopathological examination of the biopsy cores. CONCLUSION: In this study, the clinical value of PSMA-PET/CT for patients with an elevated PSA-level, and negative for PCa biopsies was low. Only very few men were diagnosed with PCa, and no clinically significant PCa was found.

Quantification of [18F]afatinib using PET/CT in NSCLC patients: a feasibility study.

INTRODUCTION: Only a subgroup of non-small cell lung cancer (NSCLC) patients benefit from treatment using epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI) such as afatinib. Tumour uptake of [18F]afatinib using positron emission tomography (PET) may identify those patients that respond to afatinib therapy. Therefore, the aim of this study was to find the optimal tracer kinetic model for quantification of [18F]afatinib uptake in NSCLC tumours. METHODS: [18F]Afatinib PET scans were performed in 10 NSCLC patients. The first patient was scanned for the purpose of dosimetry. Subsequent patients underwent a 20-min dynamic [15O]H2O PET scan (370 MBq) followed by a dynamic [18F]afatinib PET scan (342 ± 24 MBq) of 60 or 90 min. Using the Akaike information criterion (AIC), three pharmacokinetic plasma input models were evaluated with both metabolite-corrected sampler-based input and image-derived (IDIF) input functions in combination with discrete blood samples. Correlation analysis of arterial on-line sampling versus IDIF was performed. In addition, perfusion dependency and simplified measures were assessed. RESULTS: Ten patients were included. The injected activity of [18F]afatinib was 341 ± 37 MBq. Fifteen tumours could be identified in the field of view of the scanner. Based on AIC, tumour kinetics were best described using an irreversible two-tissue compartment model and a metabolite-corrected sampler-based input function (Akaike 50%). Correlation of plasma-based input functions with metabolite-corrected IDIF was very strong (r2 = 0.93). The preferred simplified uptake parameter was the tumour-to-blood ratio over the 60- to 90-min time interval (TBR60-90). Tumour uptake of [18F]afatinib was independent of perfusion. CONCLUSION: The preferred pharmacokinetic model for quantifying [18F]afatinib uptake in NSCLC tumours was the 2T3K_vb model. TBR60-90 showed excellent correlation with this model and is the best candidate simplified method. TRIAL REGISTRATION: https://eudract.ema.europa.eu/ nr 2012-002849-38.

Whole body PD-1 and PD-L1 positron emission tomography in patients with non-small-cell lung cancer.

PD-L1 immunohistochemistry correlates only moderately with patient survival and response to PD-(L)1 treatment. Heterogeneity of tumor PD-L1 expression might limit the predictive value of small biopsies. Here we show that tumor PD-L1 and PD-1 expression can be quantified non-invasively using PET-CT in patients with non-small-cell lung cancer. Whole body PD-(L)1 PET-CT reveals significant tumor tracer uptake heterogeneity both between patients, as well as within patients between different tumor lesions.

Personalizing NSCLC therapy by characterizing tumors using TKI-PET and immuno-PET.

Non-small cell lung cancer (NSCLC) therapy has entered a rapidly advancing era of precision medicine with an ever increasing number of drugs directed against a variety of specific tumor targets. Amongst these new agents, tyrosine kinase inhibitors (TKIs) and monoclonal antibodies (mAbs) are most frequently used. However, as only a sensitive subgroup of patients benefits from targeting drugs, predictive biomarkers are needed. Positron emission tomography (PET) may offer such a biomarker for predicting therapy efficacy. Some of the TKIs and mAbs that are in clinical use can be radioactively labeled and used as tracers. PET can visualize and quantify tumor specific uptake of radiolabeled targeting drugs, allowing for characterization of their pharmacokinetic behavior. In this review, the clinical potential of PET using radiolabeled TKIs (TKI-PET) and mAbs (immuno-PET) in NSCLC is discussed, and an overview is provided of the most relevant preclinical and clinical studies.

A new in vivo method to study P-glycoprotein transport in tumors and the blood-brain barrier.

Drug resistance is a major cause of chemotherapy failure in cancer treatment. One reason is the overexpression of the drug efflux pump P-glycoprotein (P-gp), involved in multidrug resistance (MDR). In vivo pharmacokinetic analysis of P-gp transport might identify the capacity of modulation by P-gp substrate modulators, such as cyclosporin A. Therefore, P-gp function was measured in vivo with positron emission tomography (PET) and [11C]verapamil as radiolabeled P-gp substrate. Studies were performed in rats bearing tumors bilaterally, a P-gp-negative small cell lung carcinoma (GLC4) and its P-gp-overexpressing subline (GLC4/P-gp). For validation, in vitro and biodistribution studies with [11C]daunorubicin and [11C]verapamil were performed. [11C]Daunorubicin and [11C]verapamil accumulation were higher in GLC4 than in GLC4/P-gp cells. These levels were increased after modulation with cyclosporin A in GLC4/P-gp. Biodistribution studies showed 159% and 185% higher levels of [11C]daunorubicin and [11C]verapamil, respectively, in GLC4 than in GLC4/P-gp tumors. After cyclosporin A, [11C]daunorubicin and [11C]verapamil content in the GLC4/P-gp tumor was raised to the level of GLC4 tumors. PET measurements demonstrated a lower [11C]verapamil content in GLC4/P-gp tumors compared with GLC4 tumors. Pretreatment with cyclosporin A increased [11C]verapamil levels in GLC4/P-gp tumors (184%) and in brains (1280%). This pharmacokinetic effect was clearly visualized with PET. These results show the feasibility of in vivo P-gp function measurement under basal conditions and after modulation in solid tumors and in the brain. Therefore, PET and radiolabeled P-gp substrates may be useful as a clinical tool to select patients who might benefit from the addition of a P-gp modulator to MDR drugs.

P-glycoprotein activity and biological response.

P-glycoprotein (P-gp) is a transmembrane drug efflux pump encoded by the MDR-1 gene in humans. Most likely P-gp protects organs against endogenous and exogenous toxins by extruding toxic compounds such as chemotherapeutics and other drugs. Many drugs are substrates for P-gp. Since P-gp is also expressed in the blood-brain barrier, P-gp substrates reach lower concentrations in the brain than in P-gp-negative tissues. Failure of response to chemotherapy of malignancies can be due to intrinsic or acquired drug resistance. Many tumors are multidrug resistant (MDR); resistant to several structurally unrelated chemotherapeutic agents. Several mechanisms are involved in MDR of which P-gp is studied most extensively. P-gp extrudes drugs out of tumor cells resulting in decreased intracellular drug concentrations, leading to the MDR phenotype. Furthermore, the MDR-1 gene exhibits several single nucleotide polymorphisms, some of which result in different transport capabilities. P-gp functionality and the effect of P-gp modulation on the pharmacokinetics of novel and established drugs can be studied in vivo by positron emission tomography (PET) using carbon-11 and fluorine-18-labeled P-gp substrates and modulators. PET may demonstrate the consequences of genetic differences on tissue pharmacokinetics. Inhibitors such as calcium-channel blockers (verapamil), cyclosporin A, ONT-093, and XR9576 can modulate the P-gp functionality. With PET the effect of P-gp modulation on the bioavailability of drugs can be investigated in humans in vivo. PET also allows the measurement of the efficacy of newly developed P-gp modulators.

Drug composition matters: the influence of carrier concentration on the radiochemical purity, hydroxyapatite affinity and in-vivo bone accumulation of the therapeutic radiopharmaceutical 188Rhenium-HEDP.

INTRODUCTION: (188)Rhenium-HEDP is an effective bone-targeting therapeutic radiopharmaceutical, for treatment of osteoblastic bone metastases. It is known that the presence of carrier (non-radioactive rhenium as ammonium perrhenate) in the reaction mixture during labeling is a prerequisite for adequate bone affinity, but little is known about the optimal carrier concentration. METHODS: We investigated the influence of carrier concentration in the formulation on the radiochemical purity, in-vitro hydroxyapatite affinity and the in-vivo bone accumulation of (188)Rhenium-HEDP in mice. RESULTS: The carrier concentration influenced hydroxyapatite binding in-vitro as well as bone accumulation in-vivo. Variation in hydroxyapatite binding with various carrier concentrations seemed to be mainly driven by variation in radiochemical purity. The in-vivo bone accumulation appeared to be more complex: satisfactory radiochemical purity and hydroxyapatite affinity did not necessarily predict acceptable bio-distribution of (188)Rhenium-HEDP. CONCLUSIONS: For development of new bisphosphonate-based radiopharmaceuticals for clinical use, human administration should not be performed without previous animal bio-distribution experiments. Furthermore, our clinical formulation of (188)Rhenium-HEDP, containing 10 µmol carrier, showed excellent bone accumulation that was comparable to other bisphosphonate-based radiopharmaceuticals, with no apparent uptake in other organs. ADVANCES IN KNOWLEDGE: Radiochemical purity and in-vitro hydroxyapatite binding are not necessarily predictive of bone accumulation of (188)Rhenium-HEDP in-vivo. IMPLICATIONS FOR PATIENT CARE: The formulation for (188)Rhenium-HEDP as developed by us for clinical use exhibits excellent bone uptake and variation in carrier concentration during preparation of this radiopharmaceutical should be avoided.

Pediatric microdose and microtracer studies using 14C in Europe.

Important information gaps remain on the efficacy and safety of drugs in children. Pediatric drug development encounters several ethical, practical, and scientific challenges. One barrier to the evaluation of medicines for children is a lack of innovative methodologies that have been adapted to the needs of children. This article presents our successful experience of pediatric microdose and microtracer studies using (14) C-labeled probes in Europe to illustrate the strengths and limitations of these approaches.

Microdosing of a Carbon-14 Labeled Protein in Healthy Volunteers Accurately Predicts Its Pharmacokinetics at Therapeutic Dosages.

Preclinical development of new biological entities (NBEs), such as human protein therapeutics, requires considerable expenditure of time and costs. Poor prediction of pharmacokinetics in humans further reduces net efficiency. In this study, we show for the first time that pharmacokinetic data of NBEs in humans can be successfully obtained early in the drug development process by the use of microdosing in a small group of healthy subjects combined with ultrasensitive accelerator mass spectrometry (AMS). After only minimal preclinical testing, we performed a first-in-human phase 0/phase 1 trial with a human recombinant therapeutic protein (RESCuing Alkaline Phosphatase, human recombinant placental alkaline phosphatase [hRESCAP]) to assess its safety and kinetics. Pharmacokinetic analysis showed dose linearity from microdose (53 µg) [(14) C]-hRESCAP to therapeutic doses (up to 5.3 mg) of the protein in healthy volunteers. This study demonstrates the value of a microdosing approach in a very small cohort for accelerating the clinical development of NBEs.

The blood-brain barrier and oncology: new insights into function and modulation.

The efficacy of chemotherapy for malignant primary or metastatic brain tumours is still poor. This is at least partly due to the presence of the blood-brain barrier (BBB). The functionality of the BBB can be explained by physicochemical features and efflux pump mechanisms. An overview of the literature is presented with emphasis on oncology. The BBB consists of capillary endothelial cells that lack fenestrations and are connected together with continuous tight junctions, with a high electrical resistance. Permeability of tight junctions can be increased in vitro by contraction of the cytoskeleton, caused by bradykinin agonists. Different efflux pumps are present in the BBB. Examples are P-glycoprotein (P-gp), organic anion transporters, (OAT) and multidrug-resistance-associated proteins (MRP)(1 and 3). These pumps act as a multi-specific efflux pump for various chemotherapeutic drugs. Experiments have shown that P-gp can be inhibited by different non-chemotherapeutic substrates such as cyclosporin A. The functionality in vivo of P-gp can be measured with positron emission tomography and [(11)C]-verapamil or with single photon emission computer tomography and(99m)Tc-sestamibi. MRP(1)and MRP(3)act as organic anion transporters that in vitro act as efflux pumps for substances that are conjugated or co-transported with glutathione and glucuronide, respectively. Methotrexate has been recently demonstrated to be transported by MRP(1)and MRP(3). Results of studies which demonstrate the clinical relevance and applicability of BBB modulators are eagerly awaited.

Monitoring interactions at ATP-dependent drug efflux pumps.

Chemotherapeutic treatment of cancer patients is often unsuccessful, due to the involvement of various mechanisms, leading to multidrug resistance (MDR). In this review, I describe the mechanisms involved in MDR. Furthermore, results obtained by imaging of P-glycoprotein (P-gp) and the multidrug resistance associated protein (MRP) are reviewed. Single photon emission computed tomography (SPECT) and positron emission tomography (PET) are unique techniques to study P-gp- and MRP-mediated transport. The radiopharmaceutical (99m)Tc-sestamibi is a substrate for both P-gp and MRP. This tracer has been used for tumor imaging in clinical studies, and to visualize blockade of P-gp mediated transport after modulation of the P-gp pump. Other (99m)Tc-radiopharmaceuticals such as (99m)Tc- tetrofosmin and several (99m)Tc-Q-complexes are also substrates for P-gp. Until now, for these compounds only results from in vitro and animal studies are available. For quantification of P-gp mediated transport with PET in vivo, several agents, such as [(11)C]colchicine, [(11)C]verapamil and [(11)C]daunorubicin have been evaluated. In vivo results suggest that these radiopharmaceuticals can be used to image P-gp function in tumors. (124)I and (76)Br radiolabeled doxorubicin analogues are also useful to examine P-gp mediated transport. Leukotrienes are specific substrates for MRP. Therefore, N-[(11)C]acetyl-leukotriene E4 provides the opportunity to study MRP function non-invasively. Results obtained with this radiopharmaceutical in MRP(2) mutated GY/TR- rats indicate visualization of MRP-mediated transport. This tracer enables to study MRP transport function abnormalities in vivo such as in Dubin-Johnson patients, who are MRP(2) gene deficient. In conclusion, it is feasible to study the functionality of MDR transporters in vivo, both with SPECT and with PET. Such imaging techniques may become an important factor in the development of novel chemotherapeutic drugs.

The distribution of drug-efflux pumps, P-gp, BCRP, MRP1 and MRP2, in the normal blood-testis barrier and in primary testicular tumours.

The drug-efflux pumps P-glycoprotein (P-gp) and multidrug resistance-associated protein 1 (MRP1) are present in the blood-testis barrier (BTB) and may hamper the delivery of cytotoxic drugs to the testis. The precise localisation of P-gp and MRP1 in testicular tissue and the presence of the efflux pumps MRP2 and breast cancer resistance protein (BCRP) in the BTB are unknown. We therefore studied the localisation of these pumps in the BTB in normal testis (n = 12), in non-seminoma (n = 10) seminoma (n = 10), and testicular lymphoma (n = 9). Slides were scored semi-quantitatively for P-gp, MRP1, MRP2 and BCRP and blood vessels with factor VIII antibody. In normal testis, P-gp and BCRP were strongly expressed by myoid cells and luminal capillary endothelial wall and P-gp also by Leydig cells. MRP1 was observed at the basal side of Sertoli cells and on Leydig cells. MRP2 was only weakly expressed by myoid cells. Seminomas and non-seminomas expressed P-gp and/or BCRP and/or MRP1, lymphomas strongly expressed P-gp, weakly expressed BCRP and did not or showed weak expression of MRP1. There was very little staining for MRP2 in the tumours. Newly formed vessels in all tumours only expressed P-gp and BCRP. P-gp, BCRP and MRP1 are present in different cell layers of the normal testis, suggesting the optimal protection of spermatogenesis. In germ cell tumours, this expression pattern may explain the chemoresistance observed to P-gp, BCRP and MRP1 substrates. In germ cell tumours and testicular lymphomas, P-gp and BCRP expression by tumour cells and by newly formed vessels may also contribute to chemoresistance. These findings underscore the importance of removing the affected testis in cases of primary germ cell tumours and testicular lymphomas, irrespective of whether the patient has already undergone chemotherapy.

Carbon-11-labeled daunorubicin and verapamil for probing P-glycoprotein in tumors with PET.

UNLABELLED: One of the mechanisms for multidrug resistance (MDR) of tumors is an overexpression of the P-glycoprotein (P-gp). The cytostatic agent daunorubicin and the modulator verapamil were labeled with 11C to probe P-gp with PET. METHODS: Carbon-11-daunorubicin was prepared from 11CCH2N2 with an aldehyde precursor, followed by hydrolysis. Carbon-11-verapamil was synthesized by 11C-methylation. Both tracers were evaluated by investigating pharmacokinetics in rats and in vitro cell kinetics using human ovarian carcinoma cells. RESULTS: Amounts of 111 MBq 11C-daunorubicin were prepared. Biodistribution studies of 11C-daunorubicin in male Wistar rats showed dose-dependent pharmacokinetics, whereas with 11C-verapamil the pharmacokinetics were dose independent. In in vitro experiments with cells, the ratio of accumulation of 11C-daunorubicin in drug sensitive/resistant cell lines was 16. Addition of verapamil resulted in increased accumulation of 11C-daunorubicin in the resistant cell line. The ratios of 11C-verapamil accumulation in drug-sensitive versus the MDR counterpart were 4-5. CONCLUSION: Carbon-11-daunorubicin and 11C-verapamil both have potential for in vivo probing of P-glycoprotein with PET.

Complete in vivo reversal of P-glycoprotein pump function in the blood-brain barrier visualized with positron emission tomography.

1. Homozygously mdr1a gene disrupted mice (mdr1a(-/-) mice) and wild type mice (mdr1a(+/+) mice) were used to develop a method for P-glycoprotein (P-gp) function imaging non-invasively and to study the effect of a P-gp reversal agent on its function in vivo. 2. [11C]verapamil (0.1 mg/kg) was administered and the changes in tissue concentrations were determined ex vivo by organ extirpation and in vivo with PET. To block P-gp function, cyclosporin A was administered. 3. Biodistribution studies revealed 9.5-fold (P < 0.001) and 3.4-fold (P < 0.001) higher [11C]verapamil in the brain and testes of mdr1a(-/-) mice than in mdr1a(+/+) mice. Cyclosporin A (25 mg/kg) increased [11C]verapamil levels in the brain and testes of mdr1a(+/+) mice in both cases 3.3-fold (P < 0.01 (brain); P < 0.001 (testes)). Fifty mg/kg cyclosporin A increased [11C]verapamil in the brain 10.6-fold (P < 0.01) and in the testes 4.1-fold (P < 0.001). No increases were found in the mdr1a(-/-) mice. This indicates complete inhibition of P-gp mediated [11C]verapamil efflux. 4. Positron camera data showed lower [11C]verapamil levels in the brain of mdr1a(+/+) mice compared to those in mdr1a(-/-) mice. [11C]verapamil accumulation in the brain of mdr1a(+/+) mice was increased by cyclosporin A to levels comparable with those in mdr1a(-/-) mice, indicating that reversal of P-gp mediated efflux can be monitored by PET. 5. We conclude that cyclosporin A can fully block the P-gp function in the blood brain barrier and the testes and that PET enables the in vivo measurement of P-gp function and reversal of its function non-invasively.

Imaging of P glycoprotein function in vivo with PET.

P glycoprotein (Pgp) is expressed on cell membranes of various organs in the body, such as the capillary endothelial cells of the brain. Furthermore, Pgp can also be expressed on the cell membrane of tumour cells. Because of Pgp-mediated efflux, tissue levels of several Pgp substrates are lower than in Pgp-negative tissues. Drug levels in Pgp-expressing organs may be increased by modulation of this Pgp-facilitated transport with several compounds, such as cyclosporin A. Up to now, the presence of drug efflux pumps in tissues could only be examined at the mRNA and protein level. However, this gives no insight into the important question of the functionality of these drug efflux pumps. Information about the transport function of Pgp and the effect of modulating this function may improve the therapeutic treatment of these patients. Positron emission tomography (PET) gives us a unique opportunity to study non-invasively (patho)physiological dynamic processes in vivo. We have therefore developed and validated a method for studying Pgp-mediated transport and its modulation in vivo with PET.