The application of receptor theory to receptor-binding and enzyme-binding oncologic radiopharmaceuticals

Positron Emission Tomography in Coronary Heart Disease

With advances in scanner technology, postprocessing techniques, and the development of novel positron emission tomography (PET) tracers, the applications of PET for the study of coronary heart disease have been gaining momentum in the last few years. Depending on the tracer and acquisition protocol, cardiac PET can be used to evaluate the atherosclerotic lesion (plaque imaging) and to assess its potential consequences—ischemic versus nonischemic (perfusion imaging) and viable versus scarred (viability imaging) myocardium. The scope of this review is to summarize the role of PET in coronary heart disease.

The quest for improving the management of breast cancer by functional imaging: The discovery and development of 16α-[18F]fluoroestradiol (FES), a PET radiotracer for the estrogen receptor, a historical review

INTRODUCTION

16α-[18F]Fluoroestradiol (FES), a PET radiotracer for the estrogen receptor (ER) in breast cancer, was the first receptor-targeted PET radiotracer for oncology and is continuing to prove its value in clinical research, antiestrogen development, and breast cancer care. The story of its conception, design, evaluation and use in clinical studies parallels the evolution of the whole field of receptor-targeted radiotracers, one greatly influenced by the research and intellectual contributions of William C. Eckelman.

METHODS AND RESULTS

The development of methods for efficient production of fluorine-18, for conversion of [18F]fluoride ion into chemically reactive form, and for its rapid and efficient incorporation into suitable estrogen precursor molecules at high molar activity, were all methodological underpinnings required for the preparation of FES. FES binds to ER with very high affinity, and its in vivo uptake by ER-dependent target tissues in animal models was efficient and selective, findings that preceded its use for PET imaging in patients with breast cancer.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

Comparisons between ER levels measured by FES-PET imaging of breast tumors with tissue-specimen ER quantification by IHC and other methods show that imaging provided improved prediction of benefit from endocrine therapies. Serial imaging of ER by FES-PET, before and after dosing patients with antiestrogens, is used to determine the efficacious dose for established antiestrogens and to facilitate clinical development of new ER antagonists. Beyond FES imaging, PET-based hormone challenge tests, which evaluate the functional status of ER by monitoring rapid changes in tumor metabolic or transcriptional activity after a brief estrogen challenge, provide highly sensitive and selective predictions of whether or not there will be a favorable response to endocrine therapies. There is sufficient interest in the clinical applications of FES that FDA approval is being sought for its wider use in breast cancer.

CONCLUSIONS

FES was the first PET probe for a receptor in cancer, and its development and clinical applications in breast cancer parallel the conceptual evolution of the whole field of receptor-binding radiotracers.

The emerging role of cell surface receptor and protein binding radiopharmaceuticals in cancer diagnostics and therapy

Targeting specific cell membrane markers for both diagnostic imaging and radionuclide therapy is a rapidly evolving field in cancer research. Some of these applications have now found a role in routine clinical practice and have been shown to have a significant impact on patient management. Several molecular targets are being investigated in ongoing clinical trials and show promise for future implementation. Advancements in molecular biology have facilitated the identification of new cancer-specific targets for radiopharmaceutical development.

Primary Preclinical and Clinical Evaluation of 68Ga-DOTA-TMVP1 as a Novel VEGFR-3 PET Imaging Radiotracer in Gynecological Cancer

PURPOSE

Tumor periphery and lymph nodes of tumor-induced lymphangiogenesis often abundantly express VEGFR-3. In our previous study, we identified a 5-amino acid peptide named TMVP1, which binds specifically to VEGFR-3. The objective of this study was to develop a novel ⁶⁸Ga-labeled TMVP1 for VEGFR-3 PET imaging and to investigate its safety, biodistribution, and tumor-localizing efficacy in xenograft tumor models and a small cohort of patients with recurrent ovarian and cervical cancer.

EXPERIMENTAL DESIGN

The DOTA-conjugated TMVP1 peptide was labeled with radionuclide ⁶⁸Ga. SPR and saturation binding assays were used for the receptor-binding studies. Gynecologic xenograft tumors were employed for small-animal PET imaging and biodistribution of ⁶⁸Ga-DOTA-TMVP1 in vivo. In the clinical study, 5 healthy volunteers and 8 patients with gynecologic cancer underwent whole-body PET/CT after being injected with ⁶⁸Ga-DOTA-TMVP1.

RESULTS

DOTA-TMVP1 was successfully labeled with ⁶⁸Ga. LECs showed higher binding capacity with ⁶⁸Ga-DOTA-TMVP1 than LEC(shVEGFR-3) and human umbilical vein endothelial cells. In mice with subcutaneous C33-A and SKOV-3 xenografts, the tracer was rapidly eliminated through the kidney to the bladder, and the small-animal PET/CT helped to clearly visualize the tumors. In patients with recurrent ovarian cancer and cervical cancer, tracer accumulation well above the background level was demonstrated in most identified sites of disease; especially with recurrent endodermal sinus tumors, the diagnostic value of ⁶⁸Ga-DOTA-TMVP1 was comparable with that of ¹⁸F-FDG PET/CT.

CONCLUSIONS

⁶⁸Ga-DOTA-TMVP1 is a potential PET tracer for imaging VEGFR-3 with favorable pharmacokinetics.

Identifying erlotinib-sensitive non-small cell lung carcinoma tumors in mice using [(11)C]erlotinib PET

Background

Non-small cell lung carcinoma (NSCLC) represents approximately 80% of lung cancer cases, and over 60% of these tumors express the epidermal growth factor receptor (EGFR). Activating mutations in the tyrosine kinase (TK) domain of the EGFR are detected in 10% to 30% of NSCLC patients, and evidence of their presence is a prerequisite for initiation of first-line therapy with selective TK inhibitors (TKIs), such as gefitinib and erlotinib. To date, the selection of candidate patients for first-line treatment with EGFR TKIs requires an invasive tumor biopsy to affirm the mutational status of the receptor. This study was designed to evaluate whether positron emission tomography (PET) of NSCLC tumor-bearing mice using [¹¹C]erlotinib could distinguish erlotinib-sensitive from erlotinib-insensitive or erlotinib-resistant tumors.

Methods

Four human NSCLC cell lines were employed, expressing either of the following forms of the EGFR: (i) the wild-type receptor (QG56 cells), (ii) a mutant with an exon 19 in-frame deletion (HCC827 cells), (iii) a mutant with the exon 21 L858R point mutation (NCI-H3255 cells), and (iv) a double mutant harboring the L858R and T790M mutations (NCI-H1975 cells). Sensitivity of each cell line to the anti-proliferative effect of erlotinib was determined in vitro. In vivo PET imaging studies following i.v. injection of [¹¹C]erlotinib were carried out in nude mice bearing subcutaneous (s.c.) xenografts of the four cell lines.

Results

Cells harboring activating mutations in the EGFR TK domain (HCC827 and NCI-H3255) were approximately 1,000- and 100-fold more sensitive to erlotinib treatment in vitro, respectively, compared to the other two cell lines. [¹¹C]Erlotinib PET scans could differentiate erlotinib-sensitive tumors from insensitive (QG56) or resistant (NCI-H1975) tumors already at 12 min after injection. Nonetheless, the uptake in HCC827 tumors was significantly higher than that in NCI-H3255, possibly reflecting differences in ATP and erlotinib affinities between the EGFR mutants.

Conclusions

[¹¹C]Erlotinib imaging in mice differentiates erlotinib-sensitive NSCLC tumors from erlotinib-insensitive or erlotinib-resistant ones.

68Ga-DOTATOC PET/CT and somatostatin receptor (sst1-sst5) expression in normal human tissue: correlation of sst2 mRNA and SUVmax

PURPOSE

By targeting somatostatin receptors (sst) radiopeptides have been established for both diagnosis and therapy. For physiologically normal human tissues the study provides a normative database of maximum standardized uptake value (SUV(max)) and sst mRNA.

METHODS

A total of 120 patients were subjected to diagnostic (68)Ga-DOTATOC positron emission tomography (PET)/CT (age range 19-83 years). SUV(max) values were measured in physiologically normal tissues defined by normal morphology, absence of surgical intervention and absence of metastatic spread during clinical follow-up. Expression of sst subtypes (sst1-sst5) was measured independently in pooled adult normal human tissue by real-time reverse transcriptase polymerase chain reaction (RT-PCR).

RESULTS

SUV(max) revealed a region-specific pattern (e.g., mean ± SD, spleen 31.1 ± 10.9, kidney 16.9 ± 5.3, liver 12.8 ± 3.6, stomach 7.0 ± 3.1, head of pancreas 6.2 ± 2.3, small bowel 4.8 ± 1.8, thyroid 4.7 ± 2.2, bone 3.9 ± 1.3, large bowel 2.9 ± 0.8, muscle 2.1 ± 0.5, parotid gland 1.9 ± 0.6, axillary lymph node 0.8 ± 0.3 and lung 0.7 ± 0.3). SUV(max) was age independent. Gender differences were evident within the thyroid (female/male: 3.7 ± 1.6/5.5 ± 2.4, p < 0.001; Mann-Whitney U test) and the pancreatic head (5.5 ± 1.9/6.9 ± 2.2, p < 0.001). The sst mRNA was widely expressed and heterogeneous, showing sst1 to be most abundant. SUV(max) values exclusively correlated with sst2 expression (r = 0.846, p < 0.001; Spearman rank correlation analysis), whereas there was no correlation of SUV(max) with the expression of the other four subtypes.

CONCLUSION

In normal human tissues (68)Ga-DOTATOC imaging has been related to the expression of sst2 at the level of mRNA. The novel normative database may improve diagnostics, monitoring and therapy of sst-expressing tumours or inflammation on a molecular basis.

Radiolabeled antiviral drugs and antibodies as virus-specific imaging probes

A number of small-molecule drugs inhibit viral replication by binding directly to virion structural proteins or to the active site of a viral enzyme, or are chemically modified by a viral enzyme before inhibiting a downstream process. Similarly, antibodies used to prevent or treat viral infections attach to epitopes on virions or on viral proteins expressed on the surface of infected cells. Such drugs and antibodies can therefore be thought of as probes for the detection of viral infections, suggesting that they might be used as radiolabeled tracers to visualize sites of viral replication by single-photon emission computed tomography (SPECT) or positron emission tomography (PET) imaging. A current example of this approach is the PET imaging of herpes simplex virus infections, in which the viral thymidine kinase phosphorylates radiolabeled thymidine analogues, trapping them within infected cells. One of many possible future applications might be the use of a radiolabeled hepatitis C protease inhibitor to image infection in animals or humans and provide a quantitative measure of viral burden. This article reviews the basic features of radionuclide imaging and the characteristics of ideal tracer molecules, and discusses how antiviral drugs and antibodies could be evaluated for their suitability as virus-specific imaging probes. The use of labeled drugs as low-dose tracers would provide an alternative application for compounds that have failed to advance to clinical use because of insufficient in vivo potency, an unsuitable pharmacokinetic profile or hepato- or nephrotoxicity.

Evaluation of [(18)F]gefitinib as a molecular imaging probe for the assessment of the epidermal growth factor receptor status in malignant tumors

PURPOSE

Gefitinib, an inhibitor of the epidermal growth factor receptor-tyrosine kinase (EGFR-TK), has shown potent effects in a subset of patients carrying specific EGFR-TK mutations in advanced non-small-cell lung cancer. In this study, we asked whether PET with [(18)F]gefitinib may be used to study noninvasively the pharmacokinetics of gefitinib in vivo and to image the EGFR status of cancer cells.

MATERIALS AND METHODS

Synthesis of [(18)F]gefitinib has been previously described. The biodistribution and metabolic stability of [(18)F]gefitinib was assessed in mice and vervet monkeys for up to 2 h post injection by both micropositron emission tomography (PET)/computed tomography (CT) scans and postmortem ex vivo tissue harvesting. Uptake levels of radiolabeled gefitinib in EGFR-expressing human cancer cell lines with various levels of EGFR expression or mutation status were evaluated both in vivo and in vitro.

RESULTS

MicroPET/CT scans in two species demonstrated a rapid and predominantly hepatobiliary clearance of [(18)F]gefitinib in vivo. However, uptake levels of radiolabeled gefitinib, both in vivo and in vitro, did not correlate with EGFR expression levels or functional status. This unexpected observation was due to high nonspecific, nonsaturable cellular uptake of gefitinib.

CONCLUSION

The biodistribution of the drug analogue [(18)F]gefitinib suggests that it may be used to assess noninvasively the pharmacokinetics of gefitinib in patients by PET imaging. This is of clinical relevance, as insufficient intratumoral drug concentrations are considered to be a factor for resistance to gefitinib therapy. However, the highly nonspecific cellular binding of [(18)F]gefitinib may preclude the use of this imaging probe for noninvasive assessment of EGFR receptor status in patients.

The automatic production of 16α-[18F]fluoroestradiol using a conventional [18F]FDG module with a disposable cassette system

We have developed a fully automatic method for the synthesis of 16alpha-[18F]fluoroestradiol ([18F]FES) using a disposable cassette system and conventional [18F]FDG module. [18F]FES was synthesized using a GE TracerLab MX module and a modified module control program. Following [18F]fluorination, we hydrolyzed the product three times with a mixture of 2N HCl and CH(3)CN. After HPLC purification, the decay corrected radiochemical yield of [18F]FES was 45.3+/-2.8%, which was stable to 98.2+/-0.2% at 6h after synthesis. This new automated synthesis method provides high and reproducible yields with the advantage of a disposable cassette system.

Evaluation of radiolabeled ML04, a putative irreversible inhibitor of epidermal growth factor receptor, as a bioprobe for PET imaging of EGFR-overexpressing tumors

Overexpression of epidermal growth factor receptor (EGFR) has been implicated in tumor development and malignancy. Evaluating the degree of EGFR expression in tumors could aid in identifying patients for EGFR-targeted therapies and in monitoring treatment. Nevertheless, no currently available assay can reliably quantify receptor content in tumors. Radiolabeled inhibitors of EGFR-TK could be developed as bioprobes for positron emission tomography imaging. Such imaging agents would not only provide a noninvasive quantitative measurement of EGFR content in tumors but also serve as radionuclide carriers for targeted radiotherapy. The potency, reversibility, selectivity and specific binding characteristics of ML04, an alleged irreversible inhibitor of EGFR, were established in vitro. The distribution of the F-18-labeled compound and the extent of EGFR-specific tumor uptake were evaluated in tumor-bearing mice. ML04 demonstrated potent, irreversible and selective inhibition of EGFR, combined with specific binding to the receptor in intact cells. In vivo distribution of the radiolabeled compound revealed tumor/blood and tumor/muscle activity uptake ratios of about 7 and 5, respectively, 3 h following administration of a radiotracer. Nevertheless, only minor EGFR-specific uptake of the compound was detected in these studies, using either EGFR-negative tumors or blocking studies as controls. To improve the in vivo performance of ML04, administration via prolonged intravenous infusion is proposed. Detailed pharmacokinetic characterization of this bioprobe could assist in the development of a kinetic model that would afford accurate measurement of EGFR content in tumors.

The tyrosine kinase inhibitor PD153035: implication of labeling position on radiometabolites formed in vitro

INTRODUCTION

The epidermal growth factor receptor is highly expressed in several types of cancers. Molecules with high affinity to its intracellular tyrosine kinase domain are being developed as in vivo imaging probes. The 4-anilinoquinazoline PD153035 has promising in vitro and in vivo properties for development as a reversible radioligand. Labeling it with carbon-11 in either of its two methoxy positions can potentially give rise to different radiometabolites and, consequently, different imaging capabilities. An evaluation of the radiotracers' metabolism was needed to determine the potential significance of the labeling position.

METHODS

PD153035 was labeled in the 6- and 7-O-methoxy positions by reacting the corresponding O-desmethyl precursors with [(11)C]methyl iodide. The two radiolabeled compounds were each incubated for 1 h with human and rat liver microsomes. At five time points, the radiolabeled metabolites were examined using radio-liquid chromatography. One metabolite was isolated and subjected to mass spectroscopic analysis.

RESULTS

A major polar metabolite was obtained in all incubations. Its molecular weight was consistent with an addition of oxygen, and its fragmentation was consistent with an N-oxidation rather than an aromatic hydroxylation. Regioselective 7-O-dealkylation was also observed, albeit in substantial amounts only in the assay using human microsomes.

CONCLUSIONS

Radiolabeling in the 7-O-methoxy position is advocated, since the labeled metabolites produced in the 7-O-demethylation are polar and probably rapidly cleared. The differences observed in the incubations with rat and human microsomes suggest that in vivo positron emission tomography studies with (11)C-labeled PD153035 in rodents may not be directly predictive for studies in humans.

In vitro and in vivo investigation of matrix metalloproteinase expression in metastatic tumor models

INTRODUCTION

Overexpression of matrix metalloproteinases (MMPs), particularly MMP-2 and MMP-9, has been correlated with poor prognosis in several cancer types including lung, colon and breast. Noninvasive detection of MMP expression might allow physicians to better determine when more aggressive cancer therapy is appropriate. The peptide CTT (CTTHWGFTLC) was identified as a selective inhibitor of MMP-2/9 that inhibits the growth of MDA-MB-435 human breast cancer xenografts.

METHODS

CTT was conjugated with the bifunctional chelator DOTA (1,4,7,10-tetraazacyclotetradecane-N,N',N'',N'''-tetraacetic acid) for radiolabeling with (64)Cu (t(1/2)=12.7 h, 17.4% beta(+), 39% beta(-)), a radionuclide suitable for positron emission tomography (PET). In vitro affinity was determined in a fluorogenic substrate assay. Tumor gelatinase targeting was evaluated in both biodistribution and microPET imaging studies.

RESULTS

Cu(II)-DOTA-CTT inhibited hMMP-2 (EC(50)=8.7 microM) and mMMP-9 (EC(50)=18.2 microM) with similar affinity to CTT (hMMP-2 EC(50)=13.2 microM; mMMP-9 EC(50)=11.0 microM). In biodistribution and microPET imaging studies, (64)Cu-DOTA-CTT was taken up by MMP-2/9-positive B16F10 murine melanoma tumors. Subsequently, imaging studies using (64)Cu-DOTA-CTT were performed on MDA-MB-435 tumor-bearing mice. With zymography, tumor MMP-2/9 expression in this model was shown to be inconsistent, resulting in microPET detection of the MDA-MB-435 tumor in only 1 of 24 imaged mice. Following limited imaging success, (64)Cu-DOTA-CTT was shown to have poor in vivo stability.

CONCLUSIONS

Despite some evidence for selective uptake of (64)Cu-DOTA-CTT by gelatinase-expressing tumors, the low affinity for MMP-2 and MMP-9 and in vivo instability make this an inadequate radioligand for in vivo tumor evaluation.

Nuclear medicine applications in molecular imaging

With the emergence of the new field of molecular imaging, there is an increasing demand for development of sensitive and safe novel imaging agents that can be rapidly translated from small animal models into patients. Nuclear medicine and positron emission tomography (PET) techniques have the ability to detect and serially monitor a variety of biologic and pathophysiologic processes, usually with tracer quantities of radiolabeled peptides, drugs, and other molecules at doses free of pharmacologic side effects, unlike the current generation of intravenous agents required for magnetic resonance (MR) and computed tomography (CT) scanning. In this article, we will review a representative sampling of the wide array of radiopharmaceuticals developed specifically for nuclear medicine radionuclide imaging that have been approved for clinical use, and those in pre-clinical trials. We will also review the existing strategies used to select the appropriate biologic markers and targets for radionuclide labeling that have been employed in the development of novel radiotracers and the imaging of small animals with new microSPECT (single photon emission computed tomography) technologies.

Automated synthesis of 16alpha-[18F]fluoroestradiol-3,17beta-disulphamate

After 16alpha-[15F]fluoroestradiol ([18F]FES) has been successfully prepared in an automated module, the synthesis of 16alpha-[18F]fluoroestradiol-3,17beta-disulphamate ([18F]FESDS) is described as a module-assisted one-pot procedure which can provide 10GBq [18F]FESDS with a radiochemical purity better than 99%. The procedure is reliable and reproducible and requires a time of about 90 min. Because of its high sulphatase-inhibitory effect [15F]FESDS is thought to be a new PET tracer to image sites of high sulphatase activity.

Labeling proteins at high specific activity using N-succinimidyl 4-[18F](fluoromethyl) benzoate

High effective specific activity N-succinimidyl 4-[18F](fluoromethyl)benzoate was prepared using a reversed phase HPLC procedure. Reversed phase HPLC removed several additional impurities not removed by normal phase HPLC, thereby increasing the effective specific activity. Small amount (< 100 micrograms) of sensitive proteins such as erythropoietin can be labeled with this reagent in high yield without aggregation.

A review of new oncotropic tracers for PET imaging

We have developed three biochemical probes to determine if they are sensitive probes of early biochemical change in a tumor. All three probes appear to have the appropriate properties for in vivo imaging, but must now be evaluated as probes for the sensitive detection of changes in early malignant disease.

Synthesis, biodistribution, and estrogen receptor scintigraphy of indium-111-diethylenetriaminepentaacetic acid-tamoxifen analogue

This study was aimed at developing a hydrophilic diethylenetriaminepentaacetic acid-tamoxifen (DTPA-Tam) analogue for use in imaging estrogen receptor positive (ER+) lesions. In rat uterine cytosol, the IC50 of DTPA-Tam conjugate was 1 microM and of tamoxifen, 2 microM. Biodistribution, autoradiography, and radionuclide imaging of 111In-DTPA-Tam in breast-tumor-bearing rats showed that tumor-to-tissue ratios increased steadily between 30 min and 48 h. The in vivo response of MCF-7 breast cancer xenografts to tamoxifen and DTPA-Tam in nude mice demonstrated that DTPA-Tam could reduce tumor growth rate. These results indicate that DTPA-Tam, a new hydrophilic ER+ ligand, might be useful in diagnosing ER+ lesions.

Radiolabeling with technetium-99m to study high-capacity and low-capacity biochemical systems

After a brief review of the history of the development of technetium-99m-labeled radiopharmaceuticals, the use of technetium chelates in high-capacity systems is discussed. The latter are used in the study of five organ systems, the kidneys, liver, bone, brain, and heart. The chemical characterization of 99mTc complexes is also reviewed, followed by discussion of the various approaches to the labeling of proteins with direct labeling, the preformed chelate approach, and the antibody chelator conjugate approach. Thereafter, the labeling of biochemicals with 99mTc for use with easily saturated sites, e.g., receptors and enzymes, is considered. Finally, attention is given to factors that affect the preparation of high specific activity, high affinity 99mTc-labeled biochemicals.