Bruce Line

Nanocarriers for Nuclear Imaging and Radiotherapy of Cancer

Several nanoscale carriers (nanoparticles, liposomes, water-soluble polymers, micelles and dendrimers) have been developed for targeted delivery of cancer diagnostic and therapeutic agents. These carriers can selectively target cancer sites and carry large payloads, thereby improving cancer detection and therapy effectiveness. Further, the combination of newer nuclear imaging techniques providing high sensitivity and spatial resolution such as dual modality imaging with positron emission tomography/computed tomography (PET/CT) and use of nanoscale devices to carry diagnostic and therapeutic radionuclides with high target specificity can enable more accurate detection, staging and therapy planning of cancer. The successful clinical applications of radiolabeled monoclonal antibodies for cancer detection and therapy bode well for the future of nanoscale carrier systems in clinical oncology. Several radiolabeled multifunctional nanocarriers have been effective in detecting and treating cancer in animal models. Nonetheless, further preclinical, clinical and long-term toxicity studies will be required to translate this technology to the care of patients with cancer. The objective of this review is to present a brief but comprehensive overview of the various nuclear imaging techniques and the use of nanocarriers to deliver radionuclides for the diagnosis and therapy of cancer.

Physical aspects of yttrium-90 microsphere therapy for nonresectable hepatic tumors

Administration of yttrium-90 microspheres via the hepatic artery is an attractive approach to selectively deliver therapeutic doses of radiation to liver malignancies. This procedure allows delivering radiation absorbed doses in excess of 100 Gy to the tumors without significant liver toxicity. The microsphere therapy involves different specialties including medical oncology, radiation oncology, nuclear medicine, interventional radiology, medical physics, and radiation safety. We have treated 80 patients with nonresectable hepatic tumors with yttrium-90 microspheres during the past two years on an institutional study protocol. The nominal radiation absorbed dose to the tumor in this study was 150 Gy. Required activity was calculated based on the nominal radiation absorbed dose and patients liver volume obtained from the CT scan, assuming a uniform distribution of the microspheres within the liver. Microspheres were administered via a catheter placed into the hepatic artery. The actual radiation absorbed doses to tumors and normal liver tissue were calculated retrospectively based on the patients 99mTc-MAA study and CT scans. As expected, the activity uptake within the liver was found to be highly nonuniform and multifold tumor to nontumor uptake was observed. A partition model was used to calculate the radiation absorbed dose within each region. For a typical patient the calculated radiation absorbed doses to the tumor and liver were 402 and 118 Gy, respectively. The radiation safety procedure involves confinement of the source and proper disposal of the contaminated materials. The average exposure rates at 1 m from the patients and on contact just anterior to the liver were 6 and 135 uSv/h, respectively. The special physics and dosimetry protocol developed for this procedure is presented.

Technetium-99m-Labeled N-(2-hydroxypropyl) methacrylamide copolymers: synthesis, characterization, and in vivo biodistribution.

AbstractPurpose. To synthesize novel technetium-99m (99mTc)-labeled N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers and characterize the effect of charge and molecular weight on their biodistribution in SCID mice. Methods. Electronegative and neutral 7-kDa, 21-kDa, and 70-kDa HPMA copolymers containing a 99mTc chelating comonomer, bearing N-ω-bis(2-pyridylmethyl)-L- lysine (DPK), were synthesized by free-radical precipitation copolymerization. The copolymers were labeled via 99mTc tricarbonyl chelation to DPK-bearing comonomer. They were characterized by side-chain content, molecular weight, molecular weight distribution, radiochemical purity, and labeling stability. Scintigraphic images were obtained during the first 90 min and at 24 h postintravenous injection in SCID mice. At 24 h, organ radioactivity was determined from necropsy tissue counting. Results. 99mTc-labeled HPMA copolymers showed greater than 90% stability over a 24-h challenge with cysteine and histidine. Scintigraphic images and the necropsy data showed that the negatively charged copolymers were eliminated from the body significantly faster than the neutral copolymers in a size-dependent manner. Conclusions. To facilitate clinical scintigraphic imaging, stable chelation of 99mTc may be achieved by incorporation of a DPK-bearing comonomer into the HPMA backbone. Electronegative and neutral 99mTc-labeled HPMA copolymers of 7, 21, and 70 kDa show significant variation in organ biodistribution in SCID mice. 99mTc-labeled HPMA copolymers could be used as diagnostic agents and to study pharmacokinetics of delivery systems based on these copolymers.

Targeted Radiopharmaceutical Therapy for Advanced Lung Cancer: Phase I Trial of Rhenium Re188 P2045, a Somatostatin Analog

Background: Both small cell and non-small cell lung cancer overexpress somatostatin receptors (SSTR). P2045 peptide is an 11-amino acid somatostatin analog that binds with high affinity to SSTR and can be labeled with Tc-99m to gauge receptor prevalence or with Re-188 for 2.1 MeV beta radiotherapy. To evaluate the safety of this approach, a phase I dose-escalation study of Re-188 P2045 in SSTR-positive lung cancer was performed. Methods: Patients were required to have stage IIIb or IV or recurrent non-small cell lung cancer or extensive stage or recurrent small cell lung cancer, performance status 0 to 1, and normal organ function. There were no limitations on the number of prior therapies. Tumor SSTR was detected with Tc-99m P2045. If positive and projected renal dose of radiation from Re-188 P2045 was less than 20 Gy, treatment with escalating doses of Re-188 P2045 was instituted. Three doses were evaluated 30 mCi/m2, 60 mCi/m2, and 90 mCi/m2. A single dose of Re-188 P2045 was allowed. Dose-limiting toxicity was defined as ≥ grade 3 nonhematologic toxicity or grade 4 hematologic toxicity. Results: Fifteen patients were enrolled. The median age was 61 years. Fourteen patients had ≥2 prior chemotherapy regimens. All were imaged with Tc-99m P2045, eight patients received Re-188 P2045. The most common toxicity was mild lymphopenia. The trial was halted at the 90 mCi/m2 level when three patients were projected to have renal radiation doses above 20 Gy. The monoclonal antibody was not determined, and no responses were seen. Five of the eight patients (62.5%, 95% CI: 24–91%) had stable disease for at least 8 weeks, all of whom entered the study with progressive disease. Median overall survival was 11.5 months. Conclusions: This trial demonstrated that Re-188 P2045 was well tolerated. Tc-99m P2045 imaging allows identification of patients who may benefit from this treatment. Although responses were not seen, survival for these heavily pretreated patients is interesting and merits further research.

The effect of cascade and coincident gamma rays on PET I-124 imaging of non-uniform activity distributions

The positron emitting nuclide 1-124 is of interest as a surrogate for 1-131 prior to radioimmunotherapy. PET imaging with 1-124 can be used to view the expected biodistribution of the therapeutic agent and to provide high resolution data for dosimetry calculations prior to treatment. 1-124 imaging is complicated by gamma rays that accompany positron emission as well as by gamma ray cascades associated with decay by electron capture. Phantom imaging experiments were performed to explore whether asymmetric, non-uniform activity distributions of 1-124 significantly affect the quantitative accuracy of 1-124 measurements. The use of a scatter correction that fits single scatter simulation results to the observed sinogram tails with both a scale factor and an amplitude offset helps to provide accurate 1-124 imaging, with relative residual errors of less than about 1%. More sophisticated modeling of gamma ray emissions associated with decay of 1-124 may not be necessary for clinical applications.