Pharmacokinetic evaluation of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide in patients by positron emission tomography

Strategic, feasibility, economic, and cultural aspects of phase 0 approaches: Is it time to change the drug development process in order to increase productivity?

Research conducted over the past 2 decades has enhanced the validity and expanded the applications of microdosing and other phase 0 approaches in drug development. Phase 0 approaches can accelerate drug development timelines and reduce attrition in clinical development by increasing the quality of candidates entering clinical development and by reducing the time to "go-no-go" decisions. This can be done by adding clinical trial data (both healthy volunteers and patients) to preclinical candidate selection, and by applying methodological and operational advantages that phase 0 have over traditional approaches. The main feature of phase 0 approaches is the limited, subtherapeutic exposure to the test article. This means a reduced risk to research volunteers, and reduced regulatory requirements, timelines, and costs of first-in-human (FIH) testing. Whereas many operational aspects of phase 0 approaches are similar to those of other early phase clinical development programs, they have some unique strategic, regulatory, ethical, feasibility, economic, and cultural aspects. Here, we provide a guidance to these operational aspects and include case studies to highlight their potential impact in a range of clinical development scenarios.

Cancer Clinical Trials: What Every Radiologist Wants to Know but Is Afraid to Ask

OBJECTIVE. The purpose of this article is to provide radiologists with a guide to the fundamental principles of oncology clinical trials. The review summarizes the evolution and structure of modern clinical trials with an emphasis on the relevance of clinical trials in the field of oncologic imaging. CONCLUSION. Understanding the structure and clinical relevance of modern clinical trials is beneficial for radiologists in the field of oncologic imaging.

Nuclear Medicine in Times of COVID-19: How Radiopharmaceuticals Could Help to Fight the Current and Future Pandemics

The emergence and global spread of COVID-19, an infectious disease caused by the novel coronavirus SARS-CoV-2, has resulted in a continuing pandemic threat to global health. Nuclear medicine techniques can be used for functional imaging of (patho)physiological processes at the cellular or molecular level and for treatment approaches based on targeted delivery of therapeutic radionuclides. Ongoing development of radiolabeling methods has significantly improved the accessibility of radiopharmaceuticals for in vivo molecular imaging or targeted radionuclide therapy, but their use for biosafety threats such as SARS-CoV-2 is restricted by the contagious nature of these agents. Here, we highlight several potential uses of nuclear medicine in the context of SARS-CoV-2 and COVID-19, many of which could also be performed in laboratories without dedicated containment measures. In addition, we provide a broad overview of experimental or repurposed SARS-CoV-2-targeting drugs and describe how radiolabeled analogs of these compounds could facilitate antiviral drug development and translation to the clinic, reduce the incidence of late-stage failures and possibly provide the basis for radionuclide-based treatment strategies. Based on the continuing threat by emerging coronaviruses and other pathogens, it is anticipated that these applications of nuclear medicine will become a more important part of future antiviral drug development and treatment.

First-in-Human Trial of Epichaperome-Targeted PET in Patients with Cancer

PURPOSE

124I-PU-H71 is an investigational first-in-class radiologic agent specific for imaging tumor epichaperome formations. The intracellular epichaperome forms under cellular stress and is a clinically validated oncotherapeutic target. We conducted a first-in-human study of microdose 124I-PU-H71 for PET to study in vivo biodistribution, pharmacokinetics, metabolism, and safety; and the feasibility of epichaperome-targeted tumor imaging.

EXPERIMENTAL DESIGN

Adult patients with cancer (n = 30) received 124I-PU-H71 tracer (201±12 MBq, <25 μg) intravenous bolus followed by PET/CT scans and blood radioassays.

RESULTS

124I-PU-H71 PET detected tumors of different cancer types (breast, lymphoma, neuroblastoma, genitourinary, gynecologic, sarcoma, and pancreas). 124I-PU-H71 was retained by tumors for several days while it cleared rapidly from bones, healthy soft tissues, and blood. Radiation dosimetry is favorable and patients suffered no adverse effects.

CONCLUSIONS

Our first-in-human results demonstrate the safety and feasibility of noninvasive in vivo detection of tumor epichaperomes using 124I-PU-H71 PET, supporting clinical development of PU-H71 and other epichaperome-targeted therapeutics.

The Unique Pharmacometrics of Small Molecule Therapeutic Drug Tracer Imaging for Clinical Oncology

Simple Summary

New clinical radiology scans using trace amounts of therapeutic cancer drugs labeled with radioisotope injected into patients can provide oncologists with fundamentally unique insights about drug delivery to tumors. This new application of radiology aims to improve how cancer drugs are used, towards improving patient outcomes. The article reviews published clinical research in this important new field.

Abstract

Translational development of radiolabeled analogues or isotopologues of small molecule therapeutic drugs as clinical imaging biomarkers for optimizing patient outcomes in targeted cancer therapy aims to address an urgent and recurring clinical need in therapeutic cancer drug development: drug- and target-specific biomarker assays that can optimize patient selection, dosing strategy, and response assessment. Imaging the in vivo tumor pharmacokinetics and biomolecular pharmacodynamics of small molecule cancer drugs offers patient- and tumor-specific data which are not available from other pharmacometric modalities. This review article examines clinical research with a growing pharmacopoeia of investigational small molecule cancer drug tracers.

Phase 0/microdosing approaches: time for mainstream application in drug development?

Phase 0 approaches - which include microdosing - evaluate subtherapeutic exposures of new drugs in first-in-human studies known as exploratory clinical trials. Recent progress extends phase 0 benefits beyond assessment of pharmacokinetics to include understanding of mechanism of action and pharmacodynamics. Phase 0 approaches have the potential to improve preclinical candidate selection and enable safer, cheaper, quicker and more informed developmental decisions. Here, we discuss phase 0 methods and applications, highlight their advantages over traditional strategies and address concerns related to extrapolation and developmental timelines. Although challenges remain, we propose that phase 0 approaches be at least considered for application in most drug development scenarios.

Use of Molecular Imaging in Clinical Drug Development: a Systematic Review

Background

Molecular imaging such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can provide the crucial pharmacokinetic-pharmacodynamic information of a drug non-invasively at an early stage of clinical drug development. Nevertheless, not much has been known how molecular imaging has been actually used in drug development studies.

Methods

We searched PubMed using such keywords as molecular imaging, PET, SPECT, drug development, and new drug, or any combination of those to select papers in English, published from January 1, 1990, to December 31, 2015. The information about the publication year, therapeutic area of a drug candidate, drug development phase, and imaging modality and utility of imaging were extracted.

Results

Of 10,264 papers initially screened, 208 papers met the eligibility criteria. The more recent the publication year, the bigger the number of papers, particularly since 2010. The two major therapeutic areas using molecular imaging to develop drugs were oncology (47.6%) and the central nervous system (CNS, 36.5%), in which efficacy (63.5%) and proof-of-concept through either receptor occupancy (RO) or other than RO (29.7%), respectively, were the primary utility of molecular imaging. PET was used 4.7 times more frequently than SPECT. Molecular imaging was most frequently used in phase I clinical trials (40.8%), whereas it was employed rarely in phase 0 or exploratory IND studies (1.4%).

Conclusions

The present study confirmed the trend that molecular imaging has been more actively employed in recent clinical drug development studies although its adoption was rather slow and rare in phase 0 studies.

Integrating Dynamic Positron Emission Tomography and Conventional Pharmacokinetic Studies to Delineate Plasma and Tumor Pharmacokinetics of FAU, a Prodrug Bioactivated by Thymidylate Synthase

FAU, a pyrimidine nucleotide analogue, is a prodrug bioactivated by intracellular thymidylate synthase to form FMAU, which is incorporated into DNA, causing cell death. This study presents a model-based approach to integrating dynamic positron emission tomography (PET) and conventional plasma pharmacokinetic studies to characterize the plasma and tissue pharmacokinetics of FAU and FMAU. Twelve cancer patients were enrolled into a phase 1 study, where conventional plasma pharmacokinetic evaluation of therapeutic FAU (50-1600 mg/m² ) and dynamic PET assessment of ¹⁸ F-FAU were performed. A parent-metabolite population pharmacokinetic model was developed to simultaneously fit PET-derived tissue data and conventional plasma pharmacokinetic data. The developed model enabled separation of PET-derived total tissue concentrations into the parent drug and metabolite components. The model provides quantitative, mechanistic insights into the bioactivation of FAU and retention of FMAU in normal and tumor tissues and has potential utility to predict tumor responsiveness to FAU treatment.

Why are we failing to implement imaging studies with radiolabelled new molecular entities in early oncology drug development?

In early drug development advanced imaging techniques can help with progressing new molecular entities (NME) to subsequent phases of drug development and thus reduce attrition. However, several organizational, operational, and regulatory hurdles pose a significant barrier, potentially limiting the impact these techniques can have on modern drug development. Positron emission tomography (PET) of radiolabelled NME is arguably the best example of a complex technique with a potential to deliver unique decision-making data in small cohorts of subjects. However, to realise this potential the impediments to timely inclusion of PET into the drug development process must be overcome. In the present paper, we discuss the value of PET imaging with radiolabelled NME during early anticancer drug development, as exemplified with one such NME. We outline the multiple hurdles and propose options on how to streamline the organizational steps for future studies.

Activating adaptive cellular mechanisms of resistance following sublethal cytotoxic chemotherapy: implications for diagnostic microdosing

As Phase 0 studies have proven to be reasonably predictive of therapeutic dose pharmacokinetics, the application of microdosing has expanded into metabolism, drug-drug interactions and now diagnostics. One potentially serious issue with this application of microdosing that has not been previously discussed is the possibility of activating cellular mechanisms of drug resistance. Here, we provide an overview of Phase 0 microdosing and drug resistance, with an emphasis on cisplatin resistance, followed by a discussion of the potential for inducing acquired resistance to platinum-based or other types of chemotherapy in cancer patients participating in Phase 0 diagnostic microdosing studies. A number of alternative approaches to diagnostic microdosing, such as the human tumor cloning assay and the use of peripheral blood mononuclear cells as a surrogate for measuring DNA adducts, are discussed that would avoid exposing cancer patients to low doses of first-line chemotherapy and the possible risk of triggering cellular mechanisms of acquired resistance. Until it has been established that diagnostic microdosing in cancer patients poses no risk of acquired drug resistance, such studies should be approached with caution.

Positron Emission Tomography as a Method for Measuring Drug Delivery to Tumors in vivo: The Example of [(11)C]docetaxel

Systemic anticancer treatments fail in a substantial number of patients. This may be caused by inadequate uptake and penetration of drugs in malignant tumors. Consequently, improvement of drug delivery to solid tumors may enhance its efficacy. Before evaluating strategies to enhance drug uptake in tumors, better understanding of drug delivery to human tumors is needed. Positron emission tomography (PET) is an imaging technique that can be used to monitor drug pharmacokinetics non-invasively in patients, based on radiolabeling these drugs with short-lived positron emitters. In this mini review, principles and potential applications of PET using radiolabeled anticancer drugs will be discussed with respect to personalized treatment planning in oncology. In particular, it will be discussed how these radiolabeled anticancer drugs could help to develop strategies for improved drug delivery to solid tumors. The development and clinical implementation of PET using radiolabeled anticancer drugs will be illustrated by validation studies of carbon-11 labeled docetaxel ([(11)C]docetaxel) in lung cancer patients.

Evaluation of the angiogenesis inhibitor KR-31831 in SKOV-3 tumor-bearing mice using (64)Cu-DOTA-VEGF(121) and microPET

KR-31831 ((2R,3R,4S)-6-amino-4-[N-(4-chloropheyl)-N-(1H-imidazol-2ylmethyl)amino]-3-hydroxyl-2-methyl-2-dimethoxymethyl-3,4-dihydro-2H-1-benzopyran), an angiogenesis inhibitor, was evaluated in tumor-bearing mice using molecular imaging technology. Pre-treatment microPET images were acquired on SKOV-3 cell-implanted nude mice after injection with (64)Cu-DOTA-VEGF(121). KR-31831 (50 mg/kg) was then injected intraperitoneally into the treatment group (n=3), while injection vehicle was injected into the control (n=4) and blocking (n=3) groups. After injections occurred daily for 28 days, all groups of mice underwent post-treatment microPET imaging after injection with (64)Cu-DOTA-VEGF(121). The post-treatment images showed high tumor uptake in the control group and reduced tumor uptake in both the blocking and treatment groups. ROI analysis of the tumor images revealed 6.25%±1.18% ID/g at 1 h, 6.55%±0.69% ID/g at 2 h, and 4.68%±0.63% ID/g at 16 h in the control group; 3.87%±0.45% ID/g at 1 h, 4.50%±0.44% ID/g at 2 h, and 3.63%±0.25% ID/g at 16 h in the blocking group; and 4.03%±0.74% ID/g at 1 h, 4.37%±0.67% ID/g at 2 h, and 3.83%±0.90% ID/g at 16 h in the treatment group. Biodistribution obtained after the post-treatment microPET imaging also demonstrated high tumor uptake (3.74%±0.27% ID/g) in the control group and reduced uptakes in both the blocking group (2.69%±0.73% ID/g, P<.05) and the treatment group (3.11%±0.25% ID/g, P<.05), which correlated well with microPET imaging data. Immunofluorescence analysis showed higher levels of VEGFR2 and CD31 expressions in tumor tissues of the control and blocking groups than in tumor tissues of the treatment group. These results suggest that the antiangiogenic activity of KR-31831 is mediated through VEGFR2 and microPET serves as a useful molecular imaging tool for evaluation of a newly developed angiogenesis inhibitor, KR-31831.

Development and experimental medicine applications of PET in oncology: a historical perspective

Nearly 90 years of scientific research have led to the use of PET and the ability to forge advances in the field of oncology. In this Historial Review we outline the key developments made with this imaging technique, including the evolution of cyclotrons and scanners, together with the associated advances made in image reconstruction, presentation, analysis of data, and radiochemistry. The applications of PET to experimental medicine are summarised, and we cover how these are related to the use and development of PET, especially in the assessment of tumour biology and pharmacology. The use of PET in clinical oncology and for tissue pharmacokinetic and pharmacodynamic studies as a means of supporting drug development is detailed. The current limitations of the technology are also discussed.

Stable isotope-resolved metabolomics and applications for drug development

Advances in analytical methodologies, principally nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS), during the last decade have made large-scale analysis of the human metabolome a reality. This is leading to the reawakening of the importance of metabolism in human diseases, particularly cancer. The metabolome is the functional readout of the genome, functional genome, and proteome; it is also an integral partner in molecular regulations for homeostasis. The interrogation of the metabolome, or metabolomics, is now being applied to numerous diseases, largely by metabolite profiling for biomarker discovery, but also in pharmacology and therapeutics. Recent advances in stable isotope tracer-based metabolomic approaches enable unambiguous tracking of individual atoms through compartmentalized metabolic networks directly in human subjects, which promises to decipher the complexity of the human metabolome at an unprecedented pace. This knowledge will revolutionize our understanding of complex human diseases, clinical diagnostics, as well as individualized therapeutics and drug response. In this review, we focus on the use of stable isotope tracers with metabolomics technologies for understanding metabolic network dynamics in both model systems and in clinical applications. Atom-resolved isotope tracing via the two major analytical platforms, NMR and MS, has the power to determine novel metabolic reprogramming in diseases, discover new drug targets, and facilitates ADME studies. We also illustrate new metabolic tracer-based imaging technologies, which enable direct visualization of metabolic processes in vivo. We further outline current practices and future requirements for biochemoinformatics development, which is an integral part of translating stable isotope-resolved metabolomics into clinical reality.

Development of radiotracers for oncology--the interface with pharmacology

There is an increasing role for positron emission tomography (PET) in oncology, particularly as a component of early phase clinical trials. As a non-invasive functional imaging modality, PET can be used to assess both pharmacokinetics and pharmacodynamics of novel therapeutics by utilizing radiolabelled compounds. These studies can provide crucial information early in the drug development process that may influence the further development of novel therapeutics. PET imaging probes can also be used as early biomarkers of clinical response and to predict clinical outcome prior to the administration of therapeutic agents. We discuss the role of PET imaging particularly as applied to phase 0 studies and discuss the regulations involved in the development and synthesis of novel radioligands. The review also discusses currently available tracers and their role in the assessment of pharmacokinetics and pharmacodynamics as applied to oncology.

Approaches using molecular imaging technology -- use of PET in clinical microdose studies

Positron emission tomography (PET) imaging uses minute amounts of radiolabeled drug tracers and thereby meets the criteria for clinical microdose studies. The advantage of PET, when compared to other analytical methods used in microdose studies, is that the pharmacokinetics (PK) of a drug can be determined in the tissue targeted for drug treatment. PET microdosing already offers interesting applications in clinical oncology and in the development of central nervous system pharmaceuticals and is extending its range of application to many other fields of pharmaceutical medicine. Although requirements for preclinical safety testing for microdose studies have been cut down by regulatory authorities, radiopharmaceuticals increasingly need to be produced under good manufacturing practice (GMP) conditions, which increases the costs of PET microdosing studies. Further challenges in PET microdosing include combining PET with other ultrasensitive analytical methods, such as accelerator mass spectrometry (AMS), to gain plasma PK data of drugs, beyond the short PET examination periods. Finally, conducting clinical PET studies with radiolabeled drugs both at micro- and therapeutic doses is encouraged to answer the question of dose linearity in clinical microdosing.

New ionization methods and miniature mass spectrometers for biomedicine: DESI imaging for cancer diagnostics and paper spray ionization for therapeutic drug monitoring

The state-of-the-art in two new ambient ionization methods for mass spectrometry, desorption electrospray ionization (DESI) and paper spray (PS), is described and their utility is illustrated with new studies on tissue imaging and biofluid analysis. DESI is an ambient ionization method that can be performed on untreated histological sections of biological tissue in the open lab environment to image lipids, fatty acids, hormones and other compounds. Paper spray is performed in the open lab too; it involves electrospraying dry blood spots or biofluid deposits from a porous medium. PS is characterized by extreme simplicity and speed: a spot of whole blood or other biofluid is analyzed directly from paper, simply by applying a high voltage to the moist paper. Both methods are being developed for use in diagnostics as a means to inform therapy. DESI imaging is applied to create molecular maps of tissue sections without prior labeling or other sample preparation. Like other methods of mass spectrometry imaging (MSI), it combines the chemical speciation of multiple analytes with information on spatial distributions. DESI imaging provides valuable information which correlates with the disease state of tissue as determined by standard histochemical methods. Positive-ion data are presented which complement previously reported negative-ion data on paired human bladder cancerous and adjacent normal tissue sections from 20 patients. These data add to the evidence already in the literature demonstrating that differences in the distributions of particular lipids contain disease-diagnostic information. Multivariate statistical analysis using principal component analysis (PCA) is used to analyze the imaging MS data, and so confirm differences between the lipid profiles of diseased and healthy tissue types. As more such data is acquired, DESI imaging has the potential to be a diagnostic tool for future cancer detection in situ; this suggests a potential role in guiding therapy in parallel with standard histochemical and immunohistological methods. The PS methodology is aimed at high-throughput clinical measurement of quantitative levels of particular therapeutic agents in blood and other biofluids. The experiment allows individual drugs to be quantified at therapeutic levels and data is presented showing quantitative drug analysis from mixtures of therapeutic drugs in whole blood. Data on cholesterol sulfate, a new possible prostate biomarker seen at elevated levels in diseased prostate tissue, but not in healthy prostate tissue in serum are reported using paper spray ionization.

Early tumor drug pharmacokinetics is influenced by tumor perfusion but not plasma drug exposure

PURPOSE

Pharmacokinetic parameters derived from plasma sampling are used as a surrogate of tumor pharmacokinetics. However, pharmacokinetics-modulating strategies do not always result in increased therapeutic efficacy. Nonsurrogacy of plasma kinetics may be due to tissue-specific factors such as tumor perfusion.

EXPERIMENTAL DESIGN

To assess the impact of tumor perfusion and plasma drug exposure on tumor pharmacokinetics, positron emission tomography studies were done with oxygen-15 radiolabeled water in 12 patients, with 6 patients undergoing positron emission tomography studies with carbon-11 radiolabeled N-[2-(dimethylamino)ethyl]acridine-4-carboxamide and the other 6 with fluorine-18 radiolabeled 5-fluorouracil.

RESULTS

We found that tumor blood flow (mL blood/mL tissue/minute) was significantly correlated to early tumor radiotracer uptake between 4 and 6 minutes [standard uptake value (SUV)4-6; rho = 0.79; P = 0.002], tumor radiotracer exposure over 10 minutes [area under the time-activity curve (AUC)0-10; predominantly parent drug; rho = 0.86; P < 0.001], and tumor radiotracer exposure over 60 minutes (AUC0-60; predominantly radiolabeled metabolites; rho = 0.80; P = 0.002). Similarly, fractional volume of distribution of radiolabeled water in tumor (Vd) was significantly correlated with SUV4-6 (rho = 0.80; P = 0.002), AUC0-10 (rho = 0.85; P < 0.001), and AUC0-60 (rho = 0.66; P = 0.02). In contrast, no correlation was observed between plasma drug or total radiotracer exposure over 60 minutes and tumor drug uptake or exposure. Tumor blood flow was significantly correlated to Vd (rho = 0.69; P = 0.014), underlying the interdependence of tumor perfusion and Vd.

CONCLUSIONS

Tumor perfusion is a key factor that influences tumor drug uptake/exposure. Tumor vasculature-targeting strategies may thus result in improved tumor drug exposure and therefore drug efficacy.

Molecular imaging in drug development

Molecular imaging can allow the non-invasive assessment of biological and biochemical processes in living subjects. Such technologies therefore have the potential to enhance our understanding of disease and drug activity during preclinical and clinical drug development, which could aid decisions to select candidates that seem most likely to be successful or to halt the development of drugs that seem likely to ultimately fail. Here, with an emphasis on oncology, we review the applications of molecular imaging in drug development, highlighting successes and identifying key challenges that need to be addressed for successful integration of molecular imaging into the drug development process.

Microdosing studies in humans: the role of positron emission tomography

Positron emission tomography (PET)-microdosing comprises the administration of a carbon-11- or fluorine-18-labelled drug candidate to human subjects in order to describe the drug's concentration-time profile in body tissues targeted for treatment. As PET microdosing involves the administration of only microgram amounts of unlabelled drug, the potential toxicological risk to human subjects is very limited. Consequently, regulatory authorities require reduced preclinical safety testing as compared with conventional phase 1 studies. Microdose studies are gaining increasing importance in clinical drug research as they have the potential to shorten time-lines and cut costs along the critical path of drug development. Current applications of PET in anticancer, anti-infective and CNS system drug research are reviewed.

Application of PET/CT in the development of novel anticancer drugs

Combined positron emission tomography/computed tomography (PET/CT) is a relatively new imaging modality, combining the functional images of PET with the anatomical information of CT. Since its commercial introduction about 5 years ago, PET/CT has become an important tool in oncology. Currently, the technique is used for primary staging and restaging of cancer patients, as well as for surgery and radiation therapy planning. The abilities of PET/CT to measure early treatment response as well as drug distribution within the body make this technique very useful in the development of novel anticancer drugs. In this paper, the recent literature on the current role of PET/CT in drug development is reviewed.

Plasma pharmacokinetic evaluation of cytotoxic agents radiolabelled with positron emitting radioisotopes

PURPOSE

This study aimed to evaluate the utility of plasma pharmacokinetic analyses of anti-cancer agents from data obtained during positron emission tomography (PET) oncology studies of radiolabelled anti-cancer agents.

PATIENTS AND METHODS

Thirteen patients were administered fluorine-18 radiolabelled 5-FU ([(18)F]5-FU) admixed with 5-FU, corresponding to a total 5-FU dose of 380-407 mg/m2 (eight patients) and 1 mg/m2 (five patients). Nine patients received 2.2-19.2 microg/m2 of carbon-11 radiolabelled N-[2-(dimethylamino)ethyl]acridine-4-carboxamide ([11C]DACA) at 1/1,000th of phase I dose, as part of phase 0 microdosing study. Radioactivity of parent drug obtained from arterial blood samples, the injected activity of the radiolabelled drug, and the total dose of injected drug were used to obtain plasma drug concentrations. Plasma pharmacokinetic parameters were estimated using model-dependent and model-independent methods.

RESULTS

5-FU plasma concentrations at therapeutic doses were above the Km and a single compartment kinetic model was best used to fit the kinetics, with a mean half-life of 8.6 min. Clearance and volumes of distribution (Vd) obtained using both model-dependent and model-independent methods were similar. Mean (SE) clearance was 1,421(144), ml min(-1) and 1,319 (119) ml min(-1) and the mean (SE) Vd was 17.3 (1.8) l and 16.3 (1.9) l by the model-independent method and model-dependent methods, respectively. In contrast, with 1 mg/m2, plasma concentrations of 5-FU were less than the Km and a two-compartment model was used to best fit the kinetics, with the mean 5-FU half-life of 6.5 min. The mean (SE) clearances obtained by the model-independent method and model-dependent methods were 3,089 (314) ml min(-1) and 2,225 (200) ml min(-1), respectively and the mean (SE) Vd were 27.9 (7.0) l and 2.3 (0.4) l, by the model independent and dependent methods, respectively. Extrapolation of AUC0-Clast to AUC0-infinity was less than 3% in both these cohort of patients. A two-compartment model with a mean half-life of 42.1 min was used to best fit the kinetics of DACA; considerable extrapolation (mean 26%) was required to obtain AUC0-infinity from AUC0-Clast. Mean (SE) clearance of DACA was 1,920 (269) ml min(-1), with the model-independent method and 1,627 (287) ml min(-1) with the model-dependent method. Similarly, Vd [mean (SE)] of DACA with the model-independent and model-dependent methods were 118 (22) l and 50 (15) l, respectively.

CONCLUSIONS

Pharmacokinetic parameters can be estimated with confidence from PET studies for agents given at therapeutic doses, whose half-lives are significantly less than the total sampling time during the scan. Tracer studies performed alone, wherein plasma levels below the Km are expected, may also provide valuable information on drug clearance for the entire range of linear kinetics. However, drugs with half-lives longer than the sampling duration are inappropriate for PET plasma pharmacokinetic evaluation.

Technology Insight: novel imaging of molecular targets is an emerging area crucial to the development of targeted drugs

Targeted drugs hold great promise for the treatment of malignant tumors; however, there are several challenges for efficient evaluation of these drugs in preclinical and clinical studies. These challenges include identifying the 'correct', biologically active concentration and dose schedule, selecting the patients likely to benefit from treatment, monitoring inhibition of the target protein or pathway, and assessing the response of the tumor to therapy. Although anatomic imaging will remain important, molecular imaging provides several new opportunities to make the process of drug development more efficient. Various techniques for molecular imaging that enable noninvasive and quantitative imaging are now available in the preclinical and clinical settings, to aid development and evaluation of new drugs for the treatment of cancer. In this Review, we discuss the integration of molecular imaging into the process of drug development and how molecular imaging can address key questions in the preclinical and clinical evaluation of new targeted drugs. Examples include imaging of the expression and inhibition of drug targets, noninvasive tissue pharmacokinetics, and early assessment of the tumor response.

Functional PET imaging in cancer drug development

New surrogate end points for monitoring response to cancer treatment are needed for both current and novel therapeutic strategies. Positron emission tomography (PET) as a functional imaging technology provides rapid, reproducible, noninvasive in vivo assessment and quantification of several biological processes targeted by anticancer therapies. PET imaging with F-18 fluorodeoxyglucose (FDG), reflecting tumor glucose metabolism, provides relevant information regarding treatment response. Changes in tumor glucose metabolism precede changes in tumor size and reflect drug effects at a cellular level. FDG-PET enables the prediction of therapy response early in the course as well as determining the viability of residual masses after completion of treatment. The assessment of novel anticancer agents will increasingly depend on functional PET imaging. Assessing responses to new biological drugs using changes in tumor size is likely an inaccurate measure of efficacy. Likewise, monitoring for drug effects using surrogate (nontumor) tissues or serial invasive testing by tumor biopsies does not provide a good correlation with overall antitumor activity. Therefore, the information derived from PET using radiolabeled biological probes provides an alternative approach to conventional structural (anatomical) imaging. PET pharmacokinetic studies will allow for the rapid assessment of novel drug biodistribution, and much smaller patient number studies before decisions on whether or not to proceed with the development of a new drug are made. Summative readouts by PET, such as drug-induced changes in tumor glucose metabolism, tumor cell proliferation and tumor perfusion and, similarly, measures of specific changes will demonstrate whether drugs are having their intended biological effects.

Ethics at phase 0: clarifying the issues

The Food and Drug Administration (FDA) and the European Agency for the Evaluation of Medicinal Products (EMEA) recently issued documents encouraging sponsors to consider microdose testing before launching Phase I trials, and many commentators predict that such methodologies will be applied more routinely in drug development. However, exploratory testing has provoked several ethical criticisms. Skeptics question the value and validity of microdose trials, and whether they present a reasonable balance of risks and benefits for subjects. Another major criticism is that such studies serve mainly commercial ends. The present article explores these and other ethical concerns for studies conducted in the oncology setting. It concludes that microdosing is not inconsistent with prevailing practices in Phase I research, and that in principle, such studies could strengthen the ethical basis for Phase I trials by providing them better evidentiary justification.

The role of PET scanning in determining pharmacoselective doses in oncology drug development

Molecular imaging is the most sensitive and specific method for measuring in vivo molecular pathways in man. Its use in oncology has developed significantly over the last 5-10 years. Molecules can be labelled with positron emitting isotopes and the emitted radiation is detected using sensitive positron emission tomography (PET) cameras. It is now possible to measure in vivo and normal tissue pharmacokinetics of anti-cancer drugs and investigate their mechanism of action. Radiolabelling of tracers can be used to measure specific pharmacodynamic endpoints and target identification. Increasing evidence shows how these technologies, when added to early drug development, can rapidly reduce the time for entry into man and early identification of mechanisms of action. With the move towards more segmented markets and identification of specific subgroups, PET's use for noninvasive biomarkers will become in- creasingly important. However, much international effort between academia and industry is required with prioritisation of development of this technology.

Clinical molecular imaging with positron emission tomography

Molecular imaging allows for the in vivo evaluation of targeted molecules and biological processes in man. Positron emission tomography (PET) is a highly sensitive and quantitative molecular imaging modality, whose utility in clinical and experimental medicine is increasing by the day. In this article, the principles of PET and its currently accepted applications in oncology, such as cancer staging, treatment response assessment and as a prognostic marker are reviewed. Further, the evolving role of PET in areas of oncology such as radiotherapy treatment planning, anti-cancer drug development and the evaluation of patho-physiological processes which drive a cell into neoplastic activity is discussed.

Molecular imaging in cancer

Medical imaging technologies have undergone explosive growth over the past few decades and now play a central role in clinical oncology. But the truly transformative power of imaging in the clinical management of cancer patients lies ahead. Today, imaging is at a crossroads, with molecularly targeted imaging agents expected to broadly expand the capabilities of conventional anatomical imaging methods. Molecular imaging will allow clinicians to not only see where a tumor is located in the body, but also to visualize the expression and activity of specific molecules (e.g., proteases and protein kinases) and biological processes (e.g., apoptosis, angiogenesis, and metastasis) that influence tumor behavior and/or response to therapy. This information is expected to have a major impact on cancer detection, individualized treatment, and drug development, as well as our understanding of how cancer arises.

Minimally invasive pharmacokinetic and pharmacodynamic technologies in hypothesis-testing clinical trials of innovative therapies

Clinical trials of new cancer drugs should ideally include measurements of parameters such as molecular target expression, pharmacokinetic (PK) behavior, and pharmacodynamic (PD) endpoints that can be linked to measures of clinical effect. Appropriate PK/PD biomarkers facilitate proof-of-concept demonstrations for target modulation; enhance the rational selection of an optimal drug dose and schedule; aid decision-making, such as whether to continue or close a drug development project; and may explain or predict clinical outcomes. In addition, measurement of PK/PD biomarkers can minimize uncertainty associated with predicting drug safety and efficacy, reduce the high levels of drug attrition during development, accelerate drug approval, and decrease the overall costs of drug development. However, there are many challenges in the development and implementation of biomarkers that probably explain their disappointingly low implementation in phase I trials. The Pharmacodynamic/Pharmacokinetic Technologies Advisory committee of Cancer Research UK has found that submissions for phase I trials of new cancer drugs in the United Kingdom often lack detailed information about PK and/or PD endpoints, which leads to suboptimal information being obtained in those trials or to delays in starting the trials while PK/PD methods are developed and validated. Minimally invasive PK/PD technologies have logistic and ethical advantages over more invasive technologies. Here we review these technologies, emphasizing magnetic resonance spectroscopy and positron emission tomography, which provide detailed functional and metabolic information. Assays that measure effects of drugs on important biologic pathways and processes are likely to be more cost-effective than those that measure specific molecular targets. Development, validation, and implementation of minimally invasive PK/PD methods are encouraged.

Imaging the pharmacokinetics of [F-18]FAU in patients with tumors: PET studies

PURPOSE

FAU (1-(2'-deoxy-2'-fluoro-beta-D: -arabinofuranosyl) uracil) can be phosphorylated by thymidine kinase, methylated by thymidylate synthase, followed by DNA incorporation and thus functions as a DNA synthesis inhibitor. This first-in-human study of [F-18]FAU was conducted in cancer patients to determine its suitability for imaging and also to understand its pharmacokinetics as a potential antineoplastic agent.

METHODS

Six patients with colorectal (n = 3) or breast cancer (n = 3) were imaged with [F-18]FAU. Serial blood and urine samples were analyzed using HPLC to determine the clearance and metabolites.

RESULTS

Imaging showed that [F-18]FAU was concentrated in breast tumors and a lymph node metastasis (tumor-to-normal-breast-tissue-ratio 3.7-4.7). FAU retention in breast tumors was significantly higher than in normal breast tissues at 60 min and retained in tumor over 2.5 h post-injection. FAU was not retained above background in colorectal tumors. Increased activity was seen in the kidney and urinary bladder due to excretion. Decreased activity was seen in the bone marrow with a mean SUV 0.6. Over 95% of activity in the blood and urine was present as intact [F-18]FAU at the end of the study.

CONCLUSIONS

Increased [F-18]FAU retention was shown in the breast tumors but not in colorectal tumors. The increased retention of FAU in the breast compared to bone marrow indicates that FAU may be useful as an unlabeled antineoplastic agent. The low retention in the marrow indicates that unlabeled FAU might lead to little marrow toxicity; however, the images were not of high contrast to consider FAU for diagnostic clinical imaging.

Positron Emission Tomography Imaging of Small Animals in Anticancer Drug Development

Positron emission tomography (PET) imaging of small animals enables researchers to bridge the gap between in vitro science and in vivo human studies. The imaging paradigm can be established and refined in animals before implementation in humans and image data related to ex vivo assays of biological activity. Small animal PET (saPET) imaging enables assessment of baseline focal pathophysiology, pharmacokinetics, biological target modulation and the efficacy of novel drugs. The potential and challenge of this technology as applied to anticancer drug development is discussed here.

Image-Guided Cancer Therapy Using PET/CT

Functional imaging with positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of malignant disease, image-guided therapy planning, and treatment monitoring. PET scanning with the radiolabeled glucose analogue (18)F-fluorodeoxyglucose ((18)FDG) is a relatively recent addition to the clinically available technology for imaging cancer, complementing the more conventional anatomical imaging modalities of computed tomography (CT) and magnetic resonance (MR). These modalities are complementary in the sense that CT provides accurate localization of organs and lesions while PET maps both normal and abnormal tissue function. When combined, the two modalities can identify and localize functional abnormalities. Attempts to align CT and PET data sets with fusion software are generally successful in the brain, whereas the remainder of the body is more challenging owing to the increased number of possible degrees of freedom between the two scans. Recently these challenges have been addressed by the introduction of the combined PET/CT scanner, a hardware-oriented approach to image fusion. With this device, accurately registered anatomical and functional images can be acquired for each patient in a single scanning session. Currently, over 400 combined PET/CT scanners are installed in medical institutions worldwide, almost all of them for the diagnosis and staging of malignant disease. However, the real impact of this technology undoubtedly will be for cancer therapy, where PET/CT images will be used to guide biopsies and assist in surgical intervention, to define target volumes for radiation therapy and optimize dose, and to monitor response to chemotherapy and establish individualized patient treatment strategies.

2-[11C]thymidine positron emission tomography as an indicator of thymidylate synthase inhibition in patients treated with AG337

BACKGROUND

Some anticancer drugs inhibit thymidylate synthase (TS), a key enzyme for thymidine nucleotide biosynthesis. Cells can compensate for depleted thymidine levels by taking up extracellular thymidine via a salvage pathway. We investigated the use of 2-[11C]thymidine positron emission tomography (PET) to measure thymidine salvage kinetics in vivo in humans.

METHODS

Five patients with advanced gastrointestinal cancer were PET scanned both before and 1 hour after oral administration of the TS inhibitor AG337 (THYMITAQ [nolatrexed]); seven control patients were scanned twice but not treated with AG337. Thymidine salvage kinetics were measured in vivo using 2-[11C]thymidine PET and spectral analysis to obtain the standardized uptake values (SUV), the area under the time-activity curve (AUC), and the fractional retention of thymidine (FRT). Changes in PET parameters between scans in the AG337-treated and control groups were compared using the Mann-Whitney U test. The relationship between AG337 exposure and AG337-induced changes in tumor FRT and in plasma deoxyuridine levels (a conventional pharmacodynamic systemic measure of TS inhibition) was examined using Spearman's regression analysis. Statistical tests were two-sided.

RESULTS

The between-scan change in FRT in patients treated with AG337 (38% increase, 95% confidence interval [CI] = 8% to 68%) was higher than that in control patients (3% increase, 95% CI = -11% to 17%) (P =.028). The level of AG337-induced increase in both 2-[11C]thymidine FRT and plasma deoxyuridine levels was statistically significantly correlated with AG337 exposure (r = 1.00, P =.01 for both).

CONCLUSIONS

AG337 administration was associated with increased tumor tracer retention that was consistent with tumor cell uptake of exogenous 2-[11C]thymidine as a result of TS inhibition. 2-[11C]Thymidine PET can be used to measure thymidine salvage kinetics directly in the tissue of interest.

Early microdose drug studies in human volunteers can minimise animal testing: Proceedings of a workshop organised by Volunteers in Research and Testing

Testing the safety and efficacy of a successful human medicine involves many laboratory animals, which can sometimes be subjected to considerable suffering and distress. Also, it is necessary to extrapolate from the test species to humans. UK and European legislation requires that Replacement, Reduction and Refinement of animal procedures (the Three Rs) are implemented wherever possible. Over the last decade, there has been substantial progress with applying in vitro and in silico methods to both drug efficacy and safety testing. This paper is a report of the discussions and recommendations arising from a workshop on the role that might be played by human volunteer studies in the very early stages of drug development. The workshop was organised in November, 2001 by Volunteers in Research and Testing, a group of individuals in the UK which launched an initiative in 1994 to identify where and how human volunteers can participate safely in biomedical studies to replace laboratory animals. It was considered that conducting pre-Phase I very low dose human studies (sub-toxic and below the dose threshold for measurable pharmacological or clinical activity) could enable drug candidates to be assessed earlier for in vivo human pharmacokinetics and metabolism. Moreover, accelerator mass spectrometry (AMS), nuclear magnetic resonance (NMR) spectroscopy and positron emission tomography (PET) are potentially useful spectrometric and imaging methods that can be used in conjunction with such human studies. Some, limited animal tests would still be required before pre-Phase I microdose studies, to take account of the potential risk posed by completely novel chemicals. The workshop recommended that very early volunteer studies using microdoses should be introduced into the drug development process in a way that does not compromise volunteer safety or the scientific quality of the resulting safety data. This should improve the selection of drug candidates and also reduce the likelihood of later candidate failure, by providing in vivo human ADME data, especially for pharmacokinetics and metabolism, at an earlier stage in drug development than is currently the case.

Use of positron emission tomography in anticancer drug development

Positron emission tomography (PET) is increasingly being used in anticancer drug development. The technique is applicable to studies of drug delivery, and where specific probes are available, to provide pharmacodynamic readouts noninvasively in patients. Mathematical modeling of the imaging data enhances the quality of information that is obtained from such studies. This section provides a review of the PET methodologies that have been used for the development of new cancer therapies. Other than imaging of radiolabeled drugs, PET modeling has found extensive application in studies with 2-[11C]thymidine, [18F]fluorodeoxyglucose, H2(15)O, C15O, and receptor ligands.

Big physics, small doses: the use of AMS and PET in human microdosing of development drugs

The process of early clinical drug development has changed little over the past 20 years despite an up to 40% failure rate associated with inappropriate drug metabolism and pharmacokinetics of candidate molecules. A new method of obtaining human metabolism data known as microdosing has been developed which will permit smarter candidate selection by taking investigational drugs into humans earlier. Microdosing depends on the availability of two ultrasensitive 'big-physics' techniques: positron emission tomography (PET) can provide pharmacodynamic information, whereas accelerator mass spectrometry (AMS) provides pharmacokinetic information. Microdosing allows safer human studies as well as reducing the use of animals in preclinical toxicology.

Use of positron emission tomography in pharmacokinetic studies to investigate therapeutic advantage in a phase I study of 120-hour intravenous infusion XR5000

PURPOSE

XR5000 (N-[2-(dimethylamino)ethyl]acridine-4-carboxamide) is a topoisomerase I and II inhibitor. Because the cytotoxicity of XR5000 increases markedly with prolonged exposure, we performed a phase I study of weekly XR5000 by 120-hour continuous infusion over 3 weeks.

PATIENTS AND METHODS

Twenty-four patients with advanced solid cancer were treated at seven dose levels (700 to 4,060 mg/m2/120 hrs) for a total of 67 cycles. Three patients underwent positron emission tomography (PET) studies at the maximum-tolerated dose (MTD) to evaluate normal tissue and tumor carbon-11 radiolabeled XR5000 ([11C]XR5000) pharmacokinetics.

RESULTS

The dose-limiting toxicity was National Cancer Institute Common Toxicity Criteria (version 1) grade 4 chest and abdominal pain affecting the single patient receiving 4,060 mg/m2/120 hours, and the MTD was 3,010 mg/m2/120 hours. Other grade 3-4 toxicities, affecting single patients at the MTD, were myelosuppression (grade 4), raised bilirubin, vomiting, and somnolence (all grade 3). There was one partial response (adenocarcinoma of unknown primary); the remainder had progressive disease. [11C]XR5000 distributed well into the three tumors studied by PET. Tumor uptake (maximum concentration or area under the concentration versus time curve [AUC]) was less than in normal tissue in which the tumors were located. Tumor exposure (AUC; mean +/- SD in m2/mL/sec) increased when [(11)C]XR5000 was administered during an infusion of XR5000 (0.242 +/- 0.4), compared with [11C]XR5000 given alone (0.209 +/- 0.04; P <.05), indicating that tumor drug exposure was not saturated [corrected].

CONCLUSION

The recommended dose for XR5000 in phase II studies is 3,010 mg/m2/120 hours. PET studies with 11C-labeled drug were feasible and demonstrated in vivo distribution into tumors. Saturation of tumor exposure was not reached at the MTD.

Applications of Positron Emission Tomography in the Development of Molecular Targeted Cancer Therapeutics

For molecular targeted cancer therapies to fulfill their promise in cancer treatment, innovative approaches are required to overcome significant obstacles that exist in the clinical development of these agents. Positron emission tomography (PET) is a functional imaging technology that allows rapid, repeated, noninvasive, in vivo assessment and quantification of many biological processes and in some cases molecular pathways targeted by these therapies. It is highly sensitive, with the capacity to detect subnanomolar concentrations of radiotracer and provides superior image resolution to conventional nuclear medicine imaging with gamma cameras. Novel PET radiotracers have been developed that allow visualisation of a variety of processes including tumour metabolism, cell proliferation, apoptosis, hypoxia and blood flow. Furthermore, specific molecular targets including cellular receptors can be identified using radiolabelled receptor ligands or specific monoclonal antibodies. Improvements in imaging technology leading to the development of small-animal PET scanners, with resolution capable of imaging commonly used mouse models of cancer, will enable PET to play an important role in preclinical proof-of-principle drug studies. Such improvements will also facilitate the validation of imaging protocols that can be readily translated to studies in humans. The greatest utility of PET in the development of molecular targeted therapeutics, however, lies in clinical studies, where PET may play a valuable role in a number of situations. These include selection of patients for therapy through noninvasive identification of the presence of specific molecular targets, pharmacokinetic studies with labelled drugs and pharmacodynamic evaluations of biological parameters to select the optimal biological dose, and assessment of response to therapies.

PET for in vivo pharmacokinetic and pharmacodynamic measurements

Positron emission tomography (PET) scanning is evolving as a unique tool for drug development in oncology for improving both the efficacy of established treatment and in evaluating novel anticancer agents. As a non-invasive functional imaging modality, PET has an unrivalled sensitivity when monitoring the pharmacokinetics and pharmacodynamics of drugs and biochemicals when radiolabelled with short living positron-emitting radioisotopes. This is of particular relevance in assessing newer molecular-targeted therapy where conventional evaluation criteria (maximum tolerated dose and tumour shrinkage for example) may be inappropriate. PET has already been applied to a wide number of drugs to demonstrate activity in vivo from standard chemotherapy such as 5-fluorouracil (5-FU) [J Clin Oncol 17 (1999) 1580], to novel molecular agents such as those involved in tumour angiogenesis [Br J Cancer 83 (2000) P6] and antivascular therapy [Proc Annu Meet Am Soc Clin Oncol 19 (2000) 179a]. This review will evaluate the achievements of PET in the drug development process, an approach that promises to facilitate the rapid translation of scientific research into current clinical practice.

Cancer Research UK procedures in manufacture and toxicology of radiotracers intended for pre-phase I positron emission tomography studies in cancer patients

Radiolabelled compounds formulated for injection (radiopharmaceuticals), are increasingly being employed in drug development studies. These can be used in tracer amounts for either pharmacokinetic or pharmacodynamic studies. Such radiotracer studies can also be carried out early in man, even prior to conventional Phase I clinical testing. The aim of this document is to describe procedures for production and safety testing of oncology radiotracers developed for imaging by positron emission tomography in cancer patients. We propose strategies for overcoming the inability to produce compounds in sufficient quantities via the radiosynthetic routes for full chemical characterisation and toxicology testing including (i) independent confirmation as far as possible that the stable compound associated with the radiopharmaceutical is identical to the non-labelled compound, (ii) animal toxicity studies with > or = 10 times (typically 100 times) the intended tracer dose in humans scaled by body surface area, and (iii) patient monitoring during the radiotracer positron emission tomography clinical trial.