Current Pharmaceutical Design - Volume 14, Issue 31, 2008

This is the fifth issue of Current Pharmaceutical Design discussing applications of PET and SPECT in drug development. The initial issue (Vol.6, No.16, 2000) described methods for measuring the deposition, biodistribution and pharmacokinetics of drugs including their interactions with specific targets. The second issue (Vol.8, No.16, 2002) focused on the interface between nuclear medicine and molecular biology. A third issue (Vol.10, No.13, 2004) identified novel areas where molecular imaging could contribute to drug design: anti-angiogenic therapy, modulation of programmed cell death, suppression of beta-amyloid plaque formation, inhibition of cerebral acetylcholinesterase and of P-glycoprotein-mediated drug transport in the blood-brain barrier, downregulation of beta-adrenoceptors by antidepressants. The fourth issue (Vol.12, No.30, 2006) gave an overview of novel targets in the human brain for which radioligands had recently been developed: norepinephrine transporters, the enzyme cyclooxygenase-2, sigma-1, nicotinic and muscarinic receptors. The current issue is a continuation of issue 4. Several authors discuss the binding of radiopharmaceuticals to inflammatory cells. Such binding can be desirable (when infection imaging is the aim of patient scanning), but also be undesirable (when tumor detection is the primary aim). Dr. Doorduin and co-workers from the University of Groningen (The Netherlands) describe the efforts of several research groups to develop radioligands for peripheral benzodiazepine receptors (PBR) [1]. This target, which is currently called the mitochondrial 18 kD translocator protein (TSPO), is strongly overexpressed in activated microglia. PET imaging with specific PBR ligands can quantify neuroinflammation which plays a central role in the progression of neurodegenerative diseases. The PET technique can not only be employed to monitor disease progression, but also be an important tool in drug development, since therapy response and optimal drug doses can be non-invasively assessed. Dr. Chianelli and co-workers from the University of Rome Sapienza (Italy) and Radboud University, Nijmegen (The Netherlands) discuss the development of radioligands for SPECT imaging of infection [2]. Such ligands include peptides, human polyclonal antibodies, monoclonal antibodies, antibody fragments, antimicrobial agents, antimicrobial peptides and bacteriophages, labeled with ^99mTc, ¹²³I or ¹¹¹In. An important aim of this research is the development of a receptor-specific ligand that can be used for the imaging of infection and that allows a differential diagnosis between sterile and septic inflammatory processes. SPECT imaging with suitable radioligands can not only be used for diagnostic purposes but also in drug development. Drs. Van Waarde and Elsinga (Groningen, The Netherlands) give an overview of efforts to develop radiopharmaceuticals with greater tumor specificity than the currently used PET tracer, ¹⁸F-FDG [3]. Accumulation of FDG in inflammatory tissue can cause false classification as a nonresponder during anti-tumor therapy. Some proliferation markers (especially labeled nucleosides and amino acids) allow a better discrimination between tumor and inflammation, but for various reasons the specificity of such tracers will never reach 100%. Proliferation markers should therefore not be considered as replacements of FDG, but rather as useful additions to the imaging arsenal which can provide additional biochemical information for response monitoring and treatment planning. Dr. Van de Wiele and co-workers from the University Hospital Ghent (Belgium) and the Unversity Medical Center Groningen (The Netherlands) introduce the subject of tumor imaging using radioligands for growth factor or peptide receptors [4]. Such receptors are coupled to intracellular signaling pathways driving tumor cell proliferation and are therefore promising targets in anti-tumor therapy. Peptide and growth factor receptor imaging can be used in the drug discovery process, to assess whether drugs are reaching tumors in sufficient amounts and how rapid they are cleared from tumor tissue. In addition, noninvasive imaging techniques may allow the selection of patients that will benefit from receptor-targeting therapies and the measurement of treatment-induced receptor downregulation. Dr.Dijkers and co-workers from the University Medical Center Groningen (The Netherlands) review PET and SPECT imaging of the human epidermal growth factor receptor (HER2/neu), using monoclonal antibodies (mAbs) and various antibody fragments, labeled with positron emitters (⁶⁴Cu, ⁶⁸Ga, ⁸⁹Zr, ⁷⁶Br,¹²⁴I) or single-photon emitters (^99mTc,¹¹¹In,¹²³I, ¹³¹I) [5]. HER2/neu is a relevant target for therapy in breast cancer. The monoclonal antibody trastuzumab (Herceptin®) and the tyrosine kinase inhibitor lapatinib (Tykerb®) have been developed to target HER2/neu. Immunoscintigraphy of HER2/neu expression may play an important role in the improvement of diagnostic imaging, the guidance of monoclonal antibody-based therapy and the development of novel mAb-based drugs.

It is important to gain more insight into neurodegenerative diseases, because these debilitating diseases can not be cured. A common characteristic of many neurological diseases is neuroinflammation, which is accompanied by the presence of activated microglia cells. In activated microglia cells, an increase in the expression of peripheral benzodiazepine receptors (PBR) can be found. The PBR was suggested as a target for monitoring disease progression and therapy efficacy with positron emission tomograpy (PET). The PET tracer [¹¹C]PK11195 has been widely used for PBR imaging, but the tracer has a high lipophilicity and high non-specific binding which makes it difficult to quantify uptake. Therefore, efforts are being made to develop more sensitive radioligands for the PBR. Animal studies have yielded several promising new tracers for PBR imaging, such as [¹¹C]DAA1106, [¹⁸F]FEDAA1106, [¹¹C]PBR28, [11C]DPA713 and [11C]CLINME. However, the potential of these new PBR ligands is still under investigation and as a consequence [¹¹C]PK11195 is used so far to image activated microglia cells in neurological disorders. With [¹¹C]PK11195, distinct neuroinflammation was detected in multiple sclerosis, Parkinson’s disease, encephalitis and other neurological diseases. Because neuroinflammation plays a central role in the progression of neurodegenerative diseases, anti-inflammatory drugs have been investigated for therapeutic intervention. Especially minocycline and cyclooxygenase inhibitors have shown in vivo anti-inflammatory, hence neuroprotective properties, that could be detected by PET imaging of the PBR with [¹¹C]PK11195. The imaging studies published so far showed that the PBR can be an important target for monitoring disease progression, therapy response and determining the optimal drug dose.

The current gold standard for imaging infection is radiolabeled white blood cells. For reasons of safety, simplicity and cost, it would be desirable to have a receptor-specific ligand that could be used for imaging infection and that would allow a differential diagnosis between sterile and septic inflammatory processes. Ligands tested for this purpose include labeled peptides (^99mTc-labeled f-Met-Leu-Phe, ¹²³I-IL-1ra, ^99mTc-IL-8, ^99mTc-P483H, ^99mTc-P1827DS, ^99mTc-C5ades- Arg, ^99mTc-RP517, 111In-DPC11870-11), human polyclonal antibodies, monoclonal antibodies, antibody fragments, antimicrobial agents (ciprofloxacin, sparfloxacin, ceftizoxime, isoniazid, ethambutol, fluconazole, all labeled with 9^99mTc), antimicrobial peptides and bacteriophages. Radiolabeled antibodies represent a valid alternative to labeled white blood cells under specific conditions and indications. Radiolabeled antibiotics and antimicrobial peptides are promising candidates for an infection-specific radiopharmaceutical. However, at present we still need to investigate many basic aspects to better understand the mechanisms of binding and accumulation of this class of radiopharmaceuticals to bacteria.

FDG, the most common radiopharmaceutical for PET imaging in oncology, is not tumor-specific. Significant tracer accumulation can also occur in viral, bacterial and fungal infections, in other forms of inflammatory tissue and in brown fat. FDG accumulation in inflammatory tissue may cause false positives during cancer screening and false classification as a nonresponder during drug treatment. Yet, discrimination between benign and malignant processes is often possible when the kinetics of FDG uptake is taken into account (e.g., by delayed, dual or dynamic PET imaging). Other PET tracers which are considered as proliferation markers may allow an improved differential diagnosis of tumor and inflammation. These include lipid precursors, amino acids, nucleosides and receptor ligands. Strictly speaking, only labelled nucleosides which are incorporated into DNA (e.g. 2-¹¹C-thymidine, ⁷⁶Br-bromofluorodeoxyuridine, ¹¹C-FMAU) are true proliferation markers, but the tissue kinetics of radiopharmaceuticals tracing amino acid transport, membrane metabolism, enzyme activity or receptor expression can be a surrogate marker of cellular proliferation if the activity of such processes is increased in rapidly dividing cells. Well-known imaging targets for oncology are: (i) glucose transport (¹⁸F-FDG); (ii) choline kinase activity (¹¹C-choline); (iii) amino acid transport (11C-methionine); and (iv) activity of thymidine kinase 1 (¹⁸F-FLT). Radiolabeled choline, amino acids and nucleosides have been reported to show greater tumor-specificity than 18F-FDG, both in experimental animals and in humans. However, the specificity of such tracers is not absolute. 11C-choline is strongly accumulated in bacterial infections and sterile inflammation. 11C-Methionine can show high uptake in brain abscesses. ¹¹F-FLT is taken up in non-metastatic reactive lymph nodes because of reactive B-lymphocyte proliferation. Moreover, FLT-PET cannot distinguish between benign lesions showing blood-brain barrier disruption and malignant brain tumors. Although proliferation is a key factor of malignancy, cell division can also occur in benign processes, including some forms of infection and inflammation. Because of such limitations, the tumor specificity of PET will never reach 100%. Each radiolabeled proliferation marker (or surrogate marker of proliferation) has high physiological uptake in some areas of the body and the tumor uptake of these radiopharmaceuticals is often lower than that of FDG. Proliferation markers should therefore not be considered as replacements of FDG, but rather as useful additions to the imaging arsenal which can provide additional diagnostic specificity and biological information for treatment planning and response monitoring.

Receptor imaging by means of positron emission tomography (PET) and single photon emission computerized tomography (SPECT) may non-invasively address questions that are essential to the development and the clinical application of drugs targeting receptors expressed on human malignancies : is the receptor targeting drug getting to the tumor in the required concentration, is there a heterogeneity in tumor uptake, how fast is the drug cleared from the tumor and how is the receptor targeting drug metabolized. Such information may be used to assess the efficacy of strategies that aim to improve drug penetration through tumor tissue or to select compounds based on their ability to penetrate tumor tissue, thereby increasing the therapeutic index. In addition, imaging by means of PET and SPECT with receptor targeting radiopharmaceuticals may allow for the selection of patients that may benefit from receptor targeting therapies either ab initio, in the situation where the levels of receptor expression are proportional to the level of signaling via the receptor, or through sequential imaging in the situation where the level of receptor expression is not proportional to the level of signaling via the receptor and proof of downregulation of the number of receptors is required.

Many new targeted anticancer drugs have been developed. In order for these drugs to be effective, the tumor target has to be present during treatment. Currently there are only a few biomarkers available to help the physician select the appropriate targeted drug for the patient and often tumor tissue is required for biomarker assays. Immunoscintigraphy might be able to improve diagnostic imaging, to guide antibody based therapy and to support early antibody development. Many different radiopharmaceuticals have been developed and used to visualize all kind of different targets especially in oncology. Intact radiolabeled antibodies generally show high tumor uptake but low tumor-to-blood ratios, particularly at early time points. Radiolabeled antibody fragments and proteins show widely differing values for tumor uptake and tumor- to-blood contrast. One of the promising targets for visualization might be HER2/neu. HER2/neu scans may prove useful for tumor staging, guiding of targeted therapy and measuring target occupancy in early drug development. Immunoscintigraphic clinical studies performed with intact antibodies indicate that HER2/neu imaging is feasible. Additional research will be performed to prove its value and make this technique applicable on a larger scale. The aim of this review is to describe the types of radiopharmaceuticals that are available, and the potential role of immunoscintigraphy in improving diagnostic imaging, guiding monoclonal antibody (mAb)-based therapy and supporting the development of mAb-based drugs using the HER2/neu target as an example.

Cerebral cannabinoid receptor (CB1) and cannabinoid drugs constitute a vibrant field in modern medicine and pharmacology. However, the physiological and pharmacological roles played by the cannabinoid receptor in the central nervous system are still not fully understood. Positron-emission tomography (PET) is the most advanced technique for non-invasive research of cerebral receptors. Quantitative PET imaging of CB1 in animal and human brains has been limited by drawbacks of the available CB1 radioligands that manifested low specific binding, high non-specific binding and/or low brain uptake. The latest research revealed three CB1 PET radioligands ([¹¹,C]JHU75528, [¹⁸F]MK9470 and [¹¹C]MePPEP) with improved imaging properties. These compounds are now being employed for the quantitative evaluation of CB1 in human subjects with PET. Molecular imaging of the CB1 receptor with these radioligands has now become possible and their application in healthy humans and in patients is underway. Despite the substantial progress in development of CB1 PET radioligands even the latest radioligands manifest certain disadvantages. Current research efforts on the development of CB1 radioligands with higher binding potential, greater brain uptake and more optimal brain kinetics.