Carbon-11 labelling of the antitumour agent N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA) and determination of plasma metabolites in man

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.

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.

Pharmacokinetic imaging: a noninvasive method for determining drug distribution and action

Advances in positron emission tomography (PET), single photon emission computed tomography (SPECT) and magnetic resonance spectroscopy (MRS), and the ability to label a wide variety of compounds for in vivo use in humans, have created a new technology for making precise physiological and pharmacological measurements. Due to the noninvasive nature of these approaches, repetitive and/or continuous measurements have become possible. Thus far, these techniques have been primarily used for one-time assessments of individuals. However, experience suggests that a major use of this technology will be in the evaluation of new drug therapies. Already, these techniques have been used to measure precisely and noninvasively the pharmacokinetics of a variety of antimicrobial, antineoplastic and CNS agents. In the case of CNS drugs, imaging techniques (particularly PET) have been used to define the classes of neuroreceptors with which the drug interacts. The physiological, pharmacological and biochemical measurements that can be performed noninvasively using modern imaging techniques can greatly facilitate the evaluation of new therapies. These measurements are most likely to be useful during drug development in preclinical studies and in phase I/II human studies. Preclinically, new drugs can be precisely compared with standard therapies, or a series of analogues can be screened for further development on the basis of performance in animal models. In Phase I/II, imaging measurements can be combined with classical pharmacokinetic data to establish optimal administration schedules, evaluate the utility of interventions in specific clinical situations, and aid in the design of Phase III trials.

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

PURPOSE

To evaluate tumor, normal tissue, and plasma pharmacokinetics of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (DACA). The study aimed to determine the pharmacokinetics of carbon-11-labeled DACA ([11C]DACA) and evaluate the effect of pharmacologic doses of DACA on radiotracer kinetics.

PATIENTS AND METHODS

[11C]DACA (at 1/1,000 phase I starting dose) was administered to 24 patients with advanced cancer (pre-phase I) or during a phase I trial of DACA in five patients. Positron emission tomography (PET) was performed to assess pharmacokinetics and tumor blood flow. Plasma samples were analyzed for metabolite profile of [11C]DACA.

RESULTS

There was rapid systemic clearance of [11C]DACA over 60 minutes (1.57 and 1.46 L x min(-1) x m(-2) in pre-phase I and phase I studies, respectively) with the production of several radiolabeled plasma metabolites. Tumor, brain, myocardium, vertebra, spleen, liver, lung, and kidneys showed appreciable uptake of 11C radioactivity. The area under the time-versus-radioactivity curves (AUC) showed the highest variability in tumors. Of interest to potential toxicity, maximum radiotracer concentrations (Cmax) in brain and vertebra were low (0.67 and 0.54 m(2) x mL(-1), respectively) compared with other tissues. A moderate but significant correlation was observed for tumor blood flow with AUC (r = 0.76; P =.02) and standardized uptake value (SUV) at 55 minutes (r = 0.79; P =.01). A decrease in myocardial AUC ( P =.03) and splenic and myocardial SUV ( P =.01 and.004, respectively) was seen in phase I studies. Significantly higher AUC, SUV, and Cmax were observed in tumors in phase I studies.

CONCLUSION

The distribution of [11C]DACA and its radiolabeled metabolites was observed in a variety of tumors and normal tissues. In the presence of unlabeled DACA, pharmacokinetics were altered in myocardium, spleen, and tumors. These data have implications for predicting activity and toxicity of DACA and support the use of PET early in drug development.