Radiosynthesis of[11C]docetaxel

Toward prediction of efficacy of chemotherapy: a proof of concept study in lung cancer patients using [¹¹C]docetaxel and positron emission tomography

PURPOSE

Pharmacokinetics of docetaxel can be measured in vivo using positron emission tomography (PET) and a microdose of radiolabeled docetaxel ([(11)C]docetaxel). The objective of this study was to investigate whether a [(11)C]docetaxel PET microdosing study could predict tumor uptake of therapeutic doses of docetaxel.

EXPERIMENTAL DESIGN

Docetaxel-naïve lung cancer patients underwent 2 [(11)C]docetaxel PET scans; one after bolus injection of [(11)C]docetaxel and another during combined infusion of [(11)C]docetaxel and a therapeutic dose of docetaxel (75 mg·m(-2)). Compartmental and spectral analyses were used to quantify [(11)C]docetaxel tumor kinetics. [(11)C]docetaxel PET measurements were used to estimate the area under the curve (AUC) of docetaxel in tumors. Tumor response was evaluated using computed tomography scans.

RESULTS

Net rates of influx (Ki) of [(11)C]docetaxel in tumors were comparable during microdosing and therapeutic scans. [(11)C]docetaxel AUCTumor during the therapeutic scan could be predicted reliably using an impulse response function derived from the microdosing scan together with the plasma curve of [(11)C]docetaxel during the therapeutic scan. At 90 minutes, the accumulated amount of docetaxel in tumors was less than 1% of the total infused dose of docetaxel. [(11)C]docetaxel Ki derived from the microdosing scan correlated with AUCTumor of docetaxel (Spearman ρ = 0.715; P = 0.004) during the therapeutic scan and with tumor response to docetaxel therapy (Spearman ρ = -0.800; P = 0.010).

CONCLUSIONS

Microdosing data of [(11)C]docetaxel PET can be used to predict tumor uptake of docetaxel during chemotherapy. The present study provides a framework for investigating the PET microdosing concept for radiolabeled anticancer drugs in patients.

Targeting MDR in breast and lung cancer: discriminating its potential importance from the failure of drug resistance reversal studies

This special issue of Drug Resistance Updates is dedicated to multidrug resistance protein 1 (MDR-1), 35 years after its discovery. While enormous progress has been made and our understanding of drug resistance has become more sophisticated and nuanced, after 35 years the role of MDR-1 in clinical oncology remains a work in progress. Despite clear in vitro evidence that P-glycoprotein (Pgp), encoded by MDR-1, is able to dramatically reduce drug concentrations in cultured cells, and that drug accumulation can be increased by small molecule inhibitors, clinical trials testing this paradigm have mostly failed. Some have argued that it is no longer worthy of study. However, repeated analyses have demonstrated MDR-1 expression in a tumor is a poor prognostic indicator leading some to conclude MDR-1 is a marker of a more aggressive phenotype, rather than a mechanism of drug resistance. In this review we will re-evaluate the MDR-1 story in light of our new understanding of molecular targeted therapy, using breast and lung cancer as examples. In the end we will reconcile the data available and the knowledge gained in support of a thesis that we understand far more than we realize, and that we can use this knowledge to improve future therapies.

Rapid decrease in delivery of chemotherapy to tumors after anti-VEGF therapy: implications for scheduling of anti-angiogenic drugs

Current strategies combining anti-angiogenic drugs with chemotherapy provide clinical benefit in cancer patients. It is assumed that anti-angiogenic drugs, such as bevacizumab, transiently normalize abnormal tumor vasculature and contribute to improved delivery of subsequent chemotherapy. To investigate this concept, a study was performed in non-small cell lung cancer (NSCLC) patients using positron emission tomography (PET) and radiolabeled docetaxel ([(11)C]docetaxel). In NSCLC, bevacizumab reduced both perfusion and net influx rate of [(11)C]docetaxel within 5 hr. These effects persisted after 4 days. The clinical relevance of these findings is notable, as there was no evidence for a substantial improvement in drug delivery to tumors. These findings highlight the importance of drug scheduling and advocate further studies to optimize scheduling of anti-angiogenic drugs.

Absolute quantification of [(11)C]docetaxel kinetics in lung cancer patients using positron emission tomography

PURPOSE

Tumor resistance to docetaxel may be associated with reduced drug concentrations in tumor tissue. Positron emission tomography (PET) allows for quantification of radiolabeled docetaxel ([(11)C]docetaxel) kinetics and might be useful for predicting response to therapy. The primary objective was to evaluate the feasibility of quantitative [(11)C]docetaxel PET scans in lung cancer patients. The secondary objective was to investigate whether [(11)C]docetaxel kinetics were associated with tumor perfusion, tumor size, and dexamethasone administration.

EXPERIMENTAL DESIGN

Thirty-four lung cancer patients underwent dynamic PET-computed tomography (CT) scans using [(11)C]docetaxel. Blood flow was measured using oxygen-15 labeled water. The first 24 patients were premedicated with dexamethasone. For quantification of [(11)C]docetaxel kinetics, the optimal tracer kinetic model was developed and a noninvasive procedure was validated.

RESULTS

Reproducible quantification of [(11)C]docetaxel kinetics in tumors was possible using a noninvasive approach (image derived input function). Thirty-two lesions (size ≥4 cm(3)) were identified, having a variable net influx rate of [(11)C]docetaxel (range, 0.0023-0.0229 mL·cm(-3)·min(-1)). [(11)C]docetaxel uptake was highly related to tumor perfusion (Spearman's ρ = 0.815;P < 0.001), but not to tumor size (Spearman's ρ = -0.140; P = 0.446). Patients pretreated with dexamethasone showed lower [(11)C]docetaxel uptake in tumors (P = 0.013). Finally, in a subgroup of patients who subsequently received docetaxel therapy, relative high [(11)C]docetaxel uptake was related with improved tumor response.

CONCLUSIONS

Quantification of [(11)C]docetaxel kinetics in lung cancer was feasible in a clinical setting. Variable [(11)C]docetaxel kinetics in tumors may reflect differential sensitivity to docetaxel therapy. Our findings warrant further studies investigating the predictive value of [(11)C]docetaxel uptake and the effects of comedication on [(11)C]docetaxel kinetics in tumors.

Biodistribution and radiation dosimetry of 11C-labelled docetaxel in cancer patients

PURPOSE

Docetaxel is an important chemotherapeutic agent used for the treatment of several cancer types. As radiolabelled anticancer agents provide a potential means for personalized treatment planning, docetaxel was labelled with the positron emitter (11)C. Non-invasive measurements of [(11)C]docetaxel uptake in organs and tumours may provide additional information on pharmacokinetics and pharmacodynamics of the drug docetaxel. The purpose of the present study was to determine the biodistribution and radiation absorbed dose of [(11)C]docetaxel in humans.

METHODS

Biodistribution of [(11)C]docetaxel was measured in seven patients (five men and two women) with solid tumours using PET/CT. Venous blood samples were collected to measure activity in blood and plasma. Regions of interest (ROI) for various source organs were defined on PET (high [(11)C]docetaxel uptake) or CT (low [(11)C]docetaxel uptake). ROI data were used to generate time-activity curves and to calculate percentage injected dose and residence times. Radiation absorbed doses were calculated according to the MIRD method using OLINDA/EXM 1.0 software.

RESULTS

Gall bladder and liver demonstrated high [(11)C]docetaxel uptake, whilst uptake in brain and normal lung was low. The percentage injected dose at 1 h in the liver was 47 +/- 9%. [(11)C]docetaxel was rapidly cleared from plasma and no radiolabelled metabolites were detected. [(11)C]docetaxel uptake in tumours was moderate and highly variable between tumours.

CONCLUSION

The effective dose of [(11)C]docetaxel was 4.7 microSv/MBq. As uptake in normal lung is low, [(11)C]docetaxel may be a promising tracer for tumours in the thoracic region.

An N-aroyltransferase of the BAHD superfamily has broad aroyl CoA specificity in vitro with analogues of N-dearoylpaclitaxel

The native N-debenzoyl-2'-deoxypaclitaxel:N-benzoyltransferase (NDTBT), from Taxus plants, transfers a benzoyl group from the corresponding CoA thioester to the amino group of the beta-phenylalanine side chain of N-debenzoyl-2'-deoxypaclitaxel, which is purportedly on the paclitaxel (Taxol) biosynthetic pathway. To elucidate the substrate specificity of NDTBT overexpressed in Escherichia coli, the purified enzyme was incubated with semisynthetically derived N-debenzoyltaxoid substrates and aroyl CoA donors (benzoyl; ortho-, meta-, and para-substituted benzoyls; various heterole carbonyls; alkanoyls; and butenoyl), which were obtained from commercial sources or synthesized via a mixed anhydride method. Several unnatural N-aroyl-N-debenzoyl-2'-deoxypaclitaxel analogues were biocatalytically assembled with catalytic efficiencies (V(max)/K(M)) ranging between 0.15 and 1.74 nmol.min(-1).mM(-1). In addition, several N-acyl-N-debenzoylpaclitaxel variants were biosynthesized when N-debenzoylpaclitaxel and N-de(tert-butoxycarbonyl)docetaxel (i.e., 10-deacetyl-N-debenzoylpaclitaxel) were used as substrates. The relative velocity (v(rel)) for NDTBT with the latter two N-debenzoyl taxane substrates ranged between approximately 1% and 200% for the array of aroyl CoAs compared to benzoyl CoA. Interestingly, NDTBT transferred hexanoyl, acetyl, and butyryl more rapidly than butenoyl or benzoyl from the CoA donor to taxanes with isoserinoyl side chains, whereas N-debenzoyl-2'-deoxypaclitaxel was more rapidly converted to its N-benzoyl derivative than to its N-alkanoyl or N-butenoyl congeners. Biocatalytic N-acyl transfer of novel acyl groups to the amino functional group of N-debenzoylpaclitaxel and its 2'-deoxy precursor reveal the surprisingly indiscriminate specificity of this transferase. This feature of NDTBT potentially provides a tool for alternative biocatalytic N-aroylation/alkanoylation to construct next generation taxanes or other novel bioactive diterpene compounds.

Imaging multidrug resistance with 4-[18F]fluoropaclitaxel

Multidrug resistance (MDR) is a cause of treatment failure in many cancer patients. MDR refers to a phenotype whereby a tumor is resistant to a large number of natural chemotherapeutic drugs. Having prior knowledge of the presence of such resistance would decrease morbidity from unsuccessful therapy and allow for the selection of individuals who may benefit from the coadministration of MDR-inhibiting drugs. The Tc-99m-labeled single-photon-emitting radiotracers sestamibi and tetrofosmin have shown some predictive value. However, positron-emitting radiotracers, which allow for dynamic quantitative imaging, hold promise for a more accurate and specific identification of MDRtumors.MDR-expressing tumors are resistant to paclitaxel, which is commonly used as a chemotherapeutic agent. 4-[18F]Fluoropaclitaxel (FPAC) is a PET-radiolabeled analogue of paclitaxel. Preclinical studies have shown the uptake of FPAC to be inversely proportional to tumor MDR expression. FPAC PET imaging in normal volunteers shows biodistribution to be similar to that in nonhuman primates. Imaging in a breast cancer patient showed FPAC localization in a primary tumor that responded to chemotherapy, while failure to localize in mediastinal disease corresponded with only partial response.FPAC PET imaging shows promise for the noninvasive pretreatment identification of MDR-expressing tumors. While much additional work is needed, this work represents a step toward image-guided personalized medicine.