In vivo uptake of 131I-5-iodo-2-deoxyuridine by malignant tumours in man

Targeted endoradiotherapy using nucleotides

Increased cellular proliferation is an integral part of the cancer phenotype. Hence, the sustained and continued demand on supply of DNA building blocks during the DNA replication presents a potential target for therapeutic intervention. For this propose, the α and Auger electron emitting nucleotides analogs are attractive for targeted endoradiotherapy, given that DNA of malignant cells is selectively addressed. This review summarizes development and preclinical and clinical studies of endoradiotherapeutic acting nucleoside analogs with a special focus on thymidine analogs.

Radionuclide Therapy with Iodine-125 and Other Auger–Electron-Emitting Radionuclides: Experimental Models and Clinical Applications

Auger-electron emitters represent an attractive alternative to beta-particle emitters for cancer therapy if they can be placed intracellularly, especially in close proximity to (or within) nuclear DNA. Based on investigations in animal tumor models, including those for ovarian cancer, bladder cancer, and brain and spinal cord tumors, in which the thymidine analog 5-radioiodo-2'-deoxyuridine (*IUdR) has been shown to be therapeutically efficacious, it is hypothesized that iodine-125 and other Auger-electron-emitting radionuclides might be valuable in the treatment of certain malignant diseases, assuming that uptake of the radiopharmaceutical by tumor cells exceeds that by normal dividing cells. Preliminary patient studies have shown that this requirement can be met partially by the locoregional administration of the radiopharmaceutical and metabolic modulation of its uptake by tumor cells. Investigators continue to seek molecules that can carry Auger-electron emitters to nuclear DNA, especially those radionuclides with higher Auger-electron yields and varying half-lives.

Characterizing tumors using metabolic imaging: PET imaging of cellular proliferation and steroid receptors

Treatment decisions in oncology are increasingly guided by information on the biologic characteristics of tumors. Currently, patient-specific information on tumor biology is obtained from the analysis of biopsy material. Positron emission tomography (PET) provides quantitative estimates of regional biochemistry and receptor status and can overcome the sampling error and difficulty in performing serial studies inherent with biopsy. Imaging using the glucose metabolism tracer, 2 -deoxy-2- fluoro-D-glucose (FDG), has demonstrated PET's ability to guide therapy in clinical oncology. In this review, we highlight PET approaches to imaging two other aspects of tumor biology: cellular proliferation and tumor steroid receptors. We review the biochemical and biologic processes underlying the imaging, positron-emitting radiopharmaceuticals that have been developed, quantitative image-analysis considerations, and clinical studies to date. This provides a basis for evaluating future developments in these promising applications of PET metabolic imaging.

[76Br]Bromodeoxyuridine, a potential tracer for the measurement of cell proliferation by positron emission tomography, in vitro and in vivo studies in mice

5-Bromo-2'-deoxyuridine (BrUdR) labeled with 77Br and 76Br was compared with 5-iodo-2'-deoxyuridine (IUdR) labeled with 125I or 131I, first in vitro then in in vivo experiments in mice. The results showed a significantly higher incorporation of BrUdR into DNA than IUdR, which can be explained by the greater similarity (size and surface hydrophilicity of the molecules) of BrUdR to thymidine. Both tracers are dehalogenated quickly in vivo but not in vitro. Free bromide is excreted more slowly than iodide, resulting in a higher background activity level after the application of [76Br]BrUdR and compensates for the favorable DNA incorporation. 76Br has more favorable properties than 124I for imaging purposes with positron emission tomography (PET) because of a very convenient half-life (16 h vs. 4.15 days) and about double the positron yield per decay. However, the more favorable physical properties are balanced by the slower excretion and thus the estimated radiation dose is higher in the case of 76Br than 124I. Thus, both tracers, [124I]IUdR and [76Br]BrUdR are potentially suitable but not optimal to measure cell proliferation in vivo. The difference between the two tracers is small and the extrapolation from mice to human difficult, and thus it cannot be concluded if one of the tracers would be better than the other for imaging of cancer patients.

Tumor localization and systemic absorption of intravesical instillation of radio-iodinated iododeoxyuridine in patients with bladder cancer

PURPOSE

We evaluated tumor uptake and systemic distribution of intravesically instilled iododeoxyuridine (IUdR) in patients with superficial bladder cancer.

MATERIALS AND METHODS

We performed 24 intravesical instillation studies in 11 patients with a mean age of 71 years. Radio-iodinated IUdR was administered through a Foley catheter. Gamma camera imaging was done after instillation and after 5 to 7 bladder irrigations. Tumor uptake was estimated by region of interest analysis. Bladder biopsy samples and surgical tumor specimens were tested for acid insoluble (deoxyribonucleic acid incorporated) radioactivity. Blood samples were obtained and analyzed for systemic absorption.

RESULTS

Imaging was positive in all patients with bladder cancer. Average tumor uptake plus or minus standard deviation was 0.185+/-0.120% of the instilled dose. Preferential uptake of IUdR in the tumor was observed in all 6 patients undergoing tissue analysis. The tumor-to-normal bladder ratio ranged from 3.2 to 74,000 (median 202). Systemic absorption of IUdR was minimal. Blood sample analysis performed after intravesical instillation in all 11 cases revealed an average uptake of 3.2x10(-5)% instilled dose per ml. (range 0.69x10(-5) to 6.7x10(-5)) in the systemic circulation. Instillation within 24 hours after transurethral bladder tumor resection in 5 cases resulted in a higher but not dangerous average systemic uptake of 7.3x10(-4)% instilled dose per ml. (range 1.3x10(-5) to 2.6x10(-3)). Instillation 1 to 4 weeks after transurethral surgery in 8 cases resulted in no increased systemic absorption with an average blood level of 3.4+/-1.8x10(-5)% instilled dose per ml. There was no detectable distribution of radioactivity into other organs, including the thyroid. We noted no evidence of systemic toxicity in the study.

CONCLUSIONS

Intravesical instillation of radio-iodinated IUdR achieves selective localization in the bladder tumor with minimal uptake by the normal bladder and minimal systemic absorption. The use of intravesical IUdR therapy for bladder cancer appears to be promising and requires further study.

Evaluation of In-111 DTPA-paclitaxel scintigraphy to predict response on murine tumors to paclitaxel

Our goal was to determine whether scintigraphy with 111In-DTPA-paclitaxel could predict the response to chemotherapy with paclitaxel.

METHODS

Ovarian carcinoma (OCA 1), mammary carcinoma (MCA-4), fibrosarcoma (FSA) and squamous cell carcinoma (SCC VII) were inoculated into the thighs of female C3Hf/Kam mice. Mice bearing 8 mm tumors were treated with paclitaxel (40 mg/kg). The growth delay, which was defined as the time in days for tumors in the treated groups to grow from 8 to 12 mm in diameter minus the time in days for tumors in the untreated control group to reach the same size, was measured to determine the effect of paclitaxel on the tumors. Sequential scintigraphy in mice bearing 10 to 14 mm tumors was conducted at 5, 30, 60, 120, 240 min and 24 hrs postinjection of 111In-DTPA-paclitaxel (3.7MBq) or 111In-DTPA as a control tracer. The tumor uptakes (% injection dose/pixel) were determined.

RESULTS

The growth delay of OCA 1, MCA-4, FSA and SCC VII tumors was 13.6, 4.0, -0.02 and -0.28 days, respectively. In other words, OCa 1 and MCA-4 were paclitaxel-sensitive tumors, whereas FSA and SCC VII were paclitaxel-resistant tumors. The tumor uptakes at 24 hrs postinjection of In-111 DTPA paclitaxel of OCA 1, MCA-4, FSA and SCC VII were 1.0 x 10(-3), 1.6 x 10(-3), 2.2 x 10(-3) and 9.0 x 10(-3) % injection dose/pixel, respectively. There was no correlation between the response to chemotherapy with paclitaxel and the tumor uptakes of 111In-DTPA-paclitaxel.

CONCLUSIONS

Scintigraphy with 111In-DTPA-paclitaxel could not predict the response to paclitaxel chemotherapy. Although there was significant accumulation of the paclitaxel in the tumor cells, additional mechanisms must be operative for the agent to be effective against the neoplasm. 111In-DTPA-paclitaxel activity is apparently different from that of paclitaxel with Cremophor.

Modulation of [5-125I]iododeoxyuridine incorporation into tumour and normal tissue DNA by methotrexate and thymidylate synthase inhibitors

A potentially useful method for imaging of micrometastases and in situ radiotherapy, would be the incorporation of radioactive labelled iododeoxyuridine (IdU) into tumour DNA. However, there are two main problems: incorporation of the radioactive IdU into normal cells and low incorporation into tumour cells. The aim of this study was to attempt to augment the incorporation of [5-125I]iododeoxyuridine (125IdU) into tumour DNA and to improve the tumour/normal tissue ratio by the use of inhibitors (methotrexate, 5-fluorouracil, AG337, ZD 1694, benzyloxybenzyl uracil) which would prolong the metabolic half-life of the compound. Mammary tumours were induced in GR mice, which were then treated with the inhibitors and the 125IdU. The tumours and representative normal tissue were removed following sacrifice of the animals, and radioactivity within the tissues measured. Pretreatment of mammary carcinoma-bearing GR mice with methotrexate caused approximately a 3-fold increase in the incorporation of 125IdU into tumour DNA, and approximately a > or = 10-fold increase in the tumour/small intestine ratio of incorporated radioactivity. Inhibition of thymidylate synthase, the enzyme involved in IdU dehalogenation, by 5-fluorouracil plus folic acid, or by novel inhibitors AG337 and ZD1694 led to a 3- to 5-fold increase in the 125IdU incorporation. Benzyloxybenzyl uracil, an inhibitor of dihydrouracil dehydrogenase, had little effect. Treatment of tumour-bearing mice with methotrexate plus ZD1694 significantly reduced the rate of tumour growth, but addition of 125IdU (70 microCi/mouse, three daily injections) had no additional antitumour activity. In conclusion, these results do not support the hypothesis that systemic administration of 125IdU can be used for cancer therapy or for imaging purposes unless better methods are found to boost its incorporation into tumour DNA.

Biochemical modulation by 5-fluorouracil and 1-folinic acid of tumor uptake of intra-arterial 5-[123I]iodo-2'deoxyuridine in patients with liver metastases from colorectal cancer

In previous studies we demonstrated a high tumor-targeting value of the 123I-labeled thymidine analogue 5-iodo-2'-deoxyuridine (IUdR) infused intra-arterially in patients with liver metastases from colorectal cancer. In the present study we have explored the possibility of enhancing tumor uptake of [123I]IUdR, by biochemical modulation with 5-fluorouracil (5-FU) and 1-folinic acid (FA), a drug combination known to inhibit thymidylate synthetase in tumor cells. The investigation was carried out employing diagnostic imaging doses of [123I]IUdR, much lower than possible therapeutic levels. In the baseline study, [123I]IUdR was infused into the hepatic artery of patients with inoperable liver metastases from colorectal cancer, and a second infusion was performed one week later, after intra-arterial administration of 5-FU and FA. The effect was evaluated by comparing tumor uptake of [123I]IUdR in the second study with that of the baseline study. The average tumor uptake immediately after [123I]IUdR infusion was 9.1% ID in the baseline study, increasing to 14.9% ID after pretreatment with 5-FU and FA. The average enhancement in early tumor uptake of [123I]IUdR induced by biochemical modulation was 72%. This enhancement was sustained at 18 and 42 hours after infusion (stable uptake). The results encourage the pretreatment of patients with 5-FU and FA prior to radioiodinated IUdR administration and suggest its inclusion in therapeutic protocols employing IUdR labeled with 123I or 125I as a source of highly cytotoxic Auger electrons.

Prediction of brain tumor therapy response by PET

In the context of brain tumor therapy it is an important issue to assess whether positron emission tomography (PET) investigation of tumor metabolism can predict tumor response to therapy, whether the information obtained with PET is substantially different from that obtained by computed tomography (CT) and magnetic resonance (MR) imaging, and whether this information will be sufficiently useful in the management of patients to warrant the cost of PET studies. Aggressive neurosurgery, radiotherapy and adjuvant chemotherapy have become the standard of care for many patients with primary brain tumors, and the limitations of CT and MR imaging in the post-treatment period have become more apparent. Both techniques are frequently unable to differentiate between therapy-related tissue changes and progressive tumor. Two clinical situations are particularly difficult to resolve: 1) transient radiographic and clinical deterioration following intensive radiotherapy or less commonly following intensive chemotherapy, and 2) clinical deterioration in a patient who has failed initial therapy, but has stable radiographic findings following a second therapy. Available PET tracers in this context fall into the following categories of tumor biochemistry: 1) energy metabolism, 2) amino acid and protein metabolism, and 3) DNA and RNA metabolism. The use of these tracers will be described in detail below. The question is not only whether therapeutic interventions specifically alter one or more of these biochemical processes in tumors, but whether the magnitude of alterations has prognostic value with respect to clinical response and survival. Moreover, an early identification of treatment 'success' or 'failure' could significantly influence patient management by providing more objective criteria for continuing or changing a specific therapeutic strategy.