Differential cytotoxicity of [123I]IUdR, [125I]IUdR and [131I]IUdR to human glioma cells in monolayer or spheroid culture: effect of proliferative heterogeneity and radiation cross-fire

Targeted Alpha Therapy: Progress in Radionuclide Production, Radiochemistry, and Applications

This review outlines the accomplishments and potential developments of targeted alpha (α) particle therapy (TAT). It discusses the therapeutic advantages of the short and highly ionizing path of α-particle emissions; the ability of TAT to complement and provide superior efficacy over existing forms of radiotherapy; the physical decay properties and radiochemistry of common α-emitters, including ²²⁵Ac, ²¹³Bi, ²²⁴Ra, ²¹²Pb, ²²⁷Th, ²²³Ra, ²¹¹At, and ¹⁴⁹Tb; the production techniques and proper handling of α-emitters in a radiopharmacy; recent preclinical developments; ongoing and completed clinical trials; and an outlook on the future of TAT.

Genotoxic Damage to Glioblastoma Cells Treated with 6 MV X-Radiation in The Presence or Absence of Methoxy Estradiol, IUDR or Topotecan

Objective

To explore the cumulative genotoxic damage to glioblastoma (GBM) cells, grown as multicellular spheroids, following exposure to 6 MV X-rays (2 Gy, 22 Gy) with or without, 2- methoxy estradiol (2ME2), iododeoxyuridine (IUDR) or topotecan (TPT), using the Picogreen assay.

Materials and Methods

The U87MG cells cultured as spheroids were treated with 6 MV X-ray using linear accelerator. Specimens were divided into five groups and irradiated using X-ray giving the dose of 2 Gy after sequentially incubated with one of the following three drug combinations: TPT, 2-ME2/TPT, IUDR/TPT or 2ME2/IUDR/ TPT. One specimen was used as the irradiated only sample (R). The last group was also irradiated with total dose of 22 Gy (each time 2 Gy) of 6 MV X-ray in 11 fractions and treated for three times. DNA damage was evaluated using the Picogreen method in the experimental study.

Results

R/TPT treated group had more DNA damage [double strand break (DSB)/single strand break (SSB)] compared with the untreated group (P<0.05). Moreover the R/ TPT group treated with 2ME2 followed by IUDR had maximum DNA damage in spheroid GBM indicating an augmented genotoxicity in the cells. The DNA damage was induced after seven fractionated irradiation and two sequential treatments with 2ME2/IUDR/TPT. To ensure accuracy of the slope of dose response curve the fractionated radiation was calculated as 7.36 Gy with respect to α/β ratio based on biologically effective dose (BED) formulae.

Conclusion

Cells treated with 2ME2/IUDR showed more sensitivity to radiation and accumulative DNA damage. DNA damage was significantly increased when GBM cells treated with TPT ceased at S phase due to the inhibition of topoisomerase enzyme and phosphorylation of Chk1 enzyme. These results suggest that R/TPT- treated cells increase sensitivity to 2ME2 and IUDR especially when they are used together. Therefore, due to an increase in the level of DNA damage (SSB vs. DSB) and impairment of DNA repair machinery, more cell death will occur. This in turn may improve the treatment of GBM.

Monitoring tumor response with radiolabeled nucleoside analogs in a hepatoma-bearing mouse model early after doxisome(®) treatment

PURPOSE

This study aims to demonstrate that 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) positron emission tomography (PET) is a promising modality for noninvasively monitoring the therapeutic efficacy of Doxisome(®) in a subcutaneous hepatoma mouse model.

PROCEDURES

Male BALB/c nu/nu mice were inoculated with HepG2 hepatoma xenograft in the right flank. Doxisome(®) (5 mg/kg, three times a week for 2 weeks) was intravenously administrated for treatment. (18)F-FLT-microPET, biodistribution studies, and immunohistochemistry of Ki-67 were performed.

RESULTS

A significant difference (p < 0.05) in tumor volume was observed on day 5 between treated and control groups. The tumor-to-muscle ratio derived from (18)F-FLT-PET and (123)I-ICdR-microSPECT images of Doxisome(®)-treated mice dropped from 12.55 ± 0.76 to 3.81 ± 0.31 and from 2.48 ± 0.42 to 1.59 ± 0.08 after a three-dose treatment, respectively, while that of the control group remained steady. The retarded proliferation rate of treated xenograft was confirmed by Ki-67 immunohistochemistry staining.

CONCLUSIONS

This study clearly demonstrated that Doxisome(®) is an effective anti-cancer drug against the growth of HepG2 hepatoma and that (18)F-FLT-PET could provide early information of tumor response during treatment.

Evaluation of The Combined Effects of Hyperthermia, Cobalt-60 Gamma Rays and IUdR on Cultured Glioblastoma Spheroid Cells and Dosimetry Using TLD-100

OBJECTIVE

In radiation treatment, the irradiation which is effective enough to control the tumors far exceeds normal-tissues tolerance. Thus to avoid such unfavourable outcomes, some methods sensitizing the tumor cells to radiation are used. Iododeoxyuridine (IUdR) is a halogenated thymidine analogue that known to be effective as a radiosensitizer in human cancer therapy. Improving the potential efficacy of radiation therapy after combining to hyperthermia depends on the magnitude of the differential sensitization of the hyperthermic effects or on the differential cytotoxicity of the radiation effects on the tumor cells. In this study, we evaluated the combined effects of IUdR, hyperthermia and gamma rays of (60)Co on human glioblastoma spheroids culture.

MATERIALS AND METHODS

In this experimental study,the cultured spheroids with 100µm diameter were treated by 1 µM IUdR, 43°C hyperthermia for an hour and 2 Gy gamma rays, respectively. The DNA damages induced in cells were compared using alkaline comet assay method, and dosimetry was then performed by TLD-100. Comet scores were calculated as mean ± standard error of mean (SEM) using one-way ANOVA.

RESULTS

Comparison of DNA damages induced by IUdR and hyperthermia + gamma treatment showed 2.67- and 1.92-fold enhancement, respectively, as compared to the damages induced by radiation alone or radiation combined IUdR. Dosimetry results showed the accurate dose delivered to cells.

CONCLUSION

Analysis of the comet tail moments of spheroids showed that the radiation treatments combined with hyperthermia and IUdR caused significant radiosensitization when compared to related results of irradiation alone or of irradiation with IUdR. These results suggest a potential clinical advantage of combining radiation with hyperthermia and indicate effectiveness of hyperthermia treatment in inducing cytotoxicity of tumor cells.

Evaluation of (131/123)I-5-iodo-2'-deoxycytidine as a novel proliferation probe in a tumor mouse model

This study evaluated a radioiodinated deoxycytidine analog, (131)I-5-iodo-2'-deoxycytidine ([(131)I]ICdR), as a novel proliferation probe and compared it with (131)I-5-iodo-2'-deoxyuridine ([(131)I]IUdR) in a NG4TL4 sarcoma-bearing mouse model. As an imaging agent, the biological characteristics of [(123)I]IUdR is not satisfactory due to its metabolic instability and short biological half-life in vivo. With [(123)I]ICdR/SPECT it was possible to clearly delineate the tumor lesion at 1h post-injection (tumor-to-muscle ratio 7.74) in tumor-bearing mice. The results of biodistribution were consistent with those observed in scintigraphic imaging. This study demonstrated that [(131)I]ICdR is a more promising SPECT probe than [(131)I]IUdR for imaging proliferation.

Evaluation of the Combined Effect of 2ME2 and (60)Co on the Inducement of DNA Damage by IUdR in a Spheroid Model of the U87MG Glioblastoma Cancer Cell Line Using Alkaline Comet Assay

OBJECTIVE

In this study, we investigated the combined effect of 2-Methoxyestradiol (2ME2) and (60)Co on the cytogenetic damage of iododeoxyuridine (IUdR) in the spheroid model of U87MG glioblastoma cancer cell lines by alkaline comet assay.

MATERIALS AND METHODS

U87MG cells were cultured as spheroids with diameters of 350 µm. As control, the spheroids of one plate were not treated. Other cultures were pretreated with 2ME2 (250 µM) for one volume doubling time (1 VDT). After this time, the subsequent treatments were performed according to the following groups: Vehicle (this sample was not treated in the 2(nd) VDT) Treated with 2ME2 (250 µM) for 1 VDT Treated simultaneously with 2ME2 (250 µM) and IUdR (1 µM) for 1 VDT Treated with 2ME2 (250 µM) for 1 VDT then irradiated with (60)Co (2 Gy) Treated simultaneously with 2ME2 (250 µM) and IUdR (1 µM) for 1 VDT then irradiated with (60)Co (2 Gy) Then the DNA damage was evaluated using the alkaline comet assay method.

RESULTS

The results showed that 2ME2 in combination with gamma irradiation of (60)Co significantly (p<0.001) increased the DNA damage by IUdR as compared to the control group. Thus the combination of these two agents increased the cytogenetic effects of IUdR in the spheroid culture model of U87MG glioblastoma cell lines.

CONCLUSION

By inhibiting the HIF-1α protein and preventing the G0 phase arrest, 2ME2 causes an increased progression into S phase and increases the IUdR absorption. Then the DNA damage in the spheroid cells increases as the uptake of IUdR is increased. These results suggest that the combined use of 2ME2 and (60)Co can increase the radiosensitization effect of IUdR.

Radiation quality-dependent bystander effects elicited by targeted radionuclides

The efficacy of radiotherapy may be partly dependent on indirect effects, which can sterilise malignant cells that are not directly irradiated. However, little is known of the influence of these effects in targeted radionuclide treatment of cancer. We determined bystander responses generated by the uptake of radioiodinated iododeoxyuridine ([*I]IUdR) and radiohaloanalogues of meta-iodobenzylguanidine ([*I]MIBG) by noradrenaline transporter (NAT) gene-transfected tumour cells. NAT specifically accumulates MIBG. Multicellular spheroids that consisted of 5% of NAT-expressing cells, capable of the active uptake of radiopharmaceutical, were sterilised by treatment with 20 kBqmL(-1) of the alpha-emitter meta-[211At]astatobenzylguanidine ([211At]MABG). Similarly, in nude mice, retardation of the growth of tumour xenografts containing 5% NAT-positivity was observed after treatment with [131I]MIBG. To determine the effect of subcellular localisation of radiolabelled drugs, we compared the bystander effects resulting from the intracellular concentration of [131I]MIBG and [131I]IUdR (low linear energy transfer (LET) beta-emitters) as well as [123I]MIBG and [123I]IUdR (high LET Auger electron emitters). [*I]IUdR is incorporated in DNA whereas [*I]MIBG accumulates in extranuclear sites. Cells exposed to media from [131I]MIBG- or [131I]IUdR-treated cells demonstrated a dose-response relationship with respect to clonogenic cell death. In contrast, cells receiving media from cultures treated with [123I]MIBG or [123I]IUdR exhibited dose-dependent toxicity at low dose but elimination of cytotoxicity with increasing radiation dose (i.e. U-shaped survival curves). Therefore radionuclides emitting high LET radiation may elicit toxic or protective effects on neighbouring untargeted cells at low and high dose respectively. It is concluded that radiopharmaceutical-induced bystander effects may depend on LET of the decay particles but are independent of site of intracellular concentration of radionuclide.

Modulation of 5-fluorouracil cytotoxicity through thymidylate synthase and NF-κB down-regulation and its application on the radiolabelled iododeoxyuridine therapy on human hepatoma cell

The inhibition of thymidylate synthase (TS) by 5-fluorouracil (5-FU) was known to increase the incorporation of radiolabelled iododeoxyuridine (IdUrd) into DNA. The relatively non-toxic compounds such as thiol-containing antioxidant pyrrolidinodithiocarbamte (PDTC) or aromatic fatty acid phenylbutyrate (PB) had been reported to enhance the cytotoxic efficacy of 5-FU. We designed a novel strategy through triplet combination of PB, PDTC and 5-FU to increase the radiolabelled IdUrd uptake and investigated the underlying mechanisms. The growth inhibition and [(125)I]IdUrd-DNA incorporation by PB, PDTC, 5-FU in different combinations were tested on parent or p21(Waf1) transfected Hep3B cells. The combination of PB and PDTC was more effective in enhancing 5-FU cytotoxicity than either drug alone. The combination of PB/PDTC and 5-FU blocked cells in S-phase and resulted in 8.5-fold increase of radiolabelled IdUrd-DNA incorporation. The transfection of p21(Waf1) did not change the general pattern of enhancement. Intriguingly, the combination of PB and PDTC effectively down-regulated NF-kappaB and TS and prevented their up-regulation from 5-FU treatment than either drug alone through a p21(Waf1)-independent mechanism. Based on this strategy, the 3-drug combination offered potential for improved radiolabelled IdUrd molecular radiotherapy for hepatoma treatment.

Modeling Multicellular Response to Nonuniform Distributions of Radioactivity: Differences in Cellular Response to Self-Dose and Cross-Dose

Radiopharmaceuticals are distributed nonuniformly in tissue. While distributions of radioactivity often appear uniform at the organ level, in fact, microscopic examination reveals that only a fraction of the cells in tissue are labeled. Labeled cells and unlabeled cells often receive different absorbed doses depending on the extent of the nonuniformity and the characteristics of the emitted radiations. The labeled cells receive an absorbed dose from radioactivity within the cell (self-dose) as well as an absorbed dose from radioactivity in surrounding labeled cells (cross-dose). Unlabeled cells receive only a cross-dose. In recent communications, a multicellular cluster model was used to investigate the lethality of microscopic nonuniform distributions of 131I iododeoxyuridine (131IdU). For a given mean absorbed dose to the tissue, the dose response depended on the percentage of cells that were labeled. Specifically, when 1, 10 and 100% of the cells were labeled, a D37 of 6.4, 5.7 and 4.5 Gy, respectively, was observed. The reason for these differences was recently traced to differences in the cellular response to the self- and cross-doses delivered by 131IdU. Systematic isolation of the effects of self-dose resulted in a D37 of 1.2 +/- 0.3 Gy. The cross-dose component yielded a D37 of 6.4 +/- 0.5 Gy. In the present work, the overall survival of multicellular clusters containing 1, 10 and 100% labeled cells is modeled using a semi-empirical approach that uses the mean lethal self- and cross-doses and the fraction of cells labeled. There is excellent agreement between the theoretical model and the experimental data when the surviving fraction is greater than 1%. Therefore, when the distribution of 131I in tissue is nonuniform at the microscopic level, and the cellular response to self- and cross-doses differs, multicellular dosimetry can be used successfully to predict biological response, whereas the mean absorbed dose fails in this regard.

The role of heat shock protein 70 in the thermoresistance of prostate cancer cell line spheroids

Heat shock protein 70 (Hsp70), a protein induced in cells exposed to sublethal heat shock, is present in all living cells and has been highly conserved during evolution. The aim of the current study was to determine the role of heat shock proteins in the resistance of prostate carcinoma cell line spheroids to hyperthermia. In vitro, the expression of Hsp70 by the DU 145 cell line, when cultured as monolayer or multicellular spheroids, was studied using Western blotting and enzyme-linked immunosorbent assay methods. The level of Hsp70 in spheroid cultures for up to 26 days at 37 degrees C remained similar to monolayer cultures. However, in samples treated with hyperthermia at 43 degrees C for 120 min, the spheroid cultures expressed a higher level of Hsp70 as compared to monolayer culture. Under similar conditions of heat treatment, the spheroids showed more heat resistance than monolayer cultures as judged by the number of colonies that they formed in suspension cultures. The results suggest that cells cultured in multicellular spheroids showed more heat resistance as compared to monolayer cultures by producing higher levels of Hsp70.

Targeting Radiotherapy to Cancer by Gene Transfer

Targeted radionuclide therapy is an alternative method of radiation treatment which uses a tumor-seeking agent carrying a radioactive atom to deposits of tumor, wherever in the body they may be located. Recent experimental data signifies promise for the amalgamation of gene transfer with radionuclide targeting. This review encompasses aspects of the integration of gene manipulation and targeted radiotherapy, highlighting the possibilities of gene transfer to assist the targeting of cancer with low molecular weight radiopharmaceuticals.

A balanced deoxyribonucleoside mixture increased the rate of DNA incorporation of 5-[125I]Iodo-2'-deoxyuridine in glioblastoma cells

Administration of nucleotide mixtures has been shown to restore and sustain the proliferation of leukocytes and enterocytes. Since it has been suggested that cancer cells use exogenous nucleotides more efficiently than normal cells, we hypothesized that administration of nucleotide mixtures would also stimulate the proliferation of cancer cells, thereby increasing the number of cells targeted by the thymidine analog 5-[(125)I]iodo-2'-deoxyuridine ([(125)I]IUdR). We first evaluated the influence of different deoxyribonucleoside mixtures on the DNA incorporation of [(125)I]IUdR in 3 human glioblastoma cell lines. Results showed that a 4-h coincubation with a mixture of identical concentration (10 microM) of deoxyadenosine, deoxyuridine, deoxyguanosine and deoxycytidine (AUGC) increased by 8.5-, 6.2-, and 2.0-fold the rate of DNA incorporation of [(125)I]IUdR in exponentially growing LN229, U87 and U251 cells, respectively. Replacing deoxyuridine by thymidine (ATGC) reversed the effect of the mixture, whereas removing deoxyuridine allowed a mixture of 10 microM AGC to increase by 2.2-fold the rate of DNA incorporation of [(125)I]IUdR in LN229 cells. Furthermore, the rate of DNA incorporation of [(125)I]IUdR in LN229 and U87 cells was increased up to 19.9- and 9.4-fold, respectively, by extending the coincubation time with 10 microM AUGC to 9 h, and up to 40.9- and 26.8-fold by incubating confluent cells for 4 h with 10 microM AUGC. Flow cytometry analysis showed that exposure of confluent cells to AUGC increased the percentage of cells in S phase of the cell cycle. Thus, co-administration of a balanced deoxyribonucleoside mixture may improve the use of radiolabeled nucleotide analogs, such as [(125)I]IUdR, for the targeting of cancer cells.

Transfectant mosaic spheroids: a new model for evaluation of tumour cell killing in targeted radiotherapy and experimental gene therapy

BACKGROUND

We describe an in vitro tumour model for targeted radiotherapy and gene therapy that incorporates cell population heterogeneity.

MATERIALS AND METHODS

Transfectant mosaic spheroids (TMS) and transfected mosaic monolayers (TMM) are composed of two cell populations derived from a single cell line. The cells of one population were transfected with the noradrenaline transporter gene (NAT), allowing active uptake of a radiolabelled targeting agent meta-[131I]iodobenzylguanidine ([131I]MIBG); the other population of cells was derived from the same parent line and transfected with a marker gene - green fluorescent protein (GFP). After treatment with [131I]MIBG, cell kill was determined in TMM by clonogenic assay and in TMS by clonogenic assay and spheroid growth delay.

RESULTS

We have used the TMS model to assess the 'radiological bystander effect' (radiation cross-fire) conferred by the beta-emitting radiopharmaceutical [131I] MIBG whose cellular uptake is facilitated by the transfected gene encoding NAT. We show that cell killing by [131I]MIBG in both TMS and TMM cultures increased in direct proportion to the fraction of NAT-transfected cells and that the degree of cell killing against fraction transfected was greater in TMS, suggestive of a greater bystander effect in the three-dimensional culture system.

CONCLUSIONS

TMS provide a useful model for assessment of the effectiveness of targeted radiotherapy in combination with gene therapy when less than 100% of the target cell population is expressing the NAT transgene. Further, this novel model offers the unique opportunity to investigate radiation-induced bystander effects and their contribution to cell cytotoxicity in radiotherapy and other gene therapy applications.

A gene therapy/targeted radiotherapy strategy for radiation cell kill by

BACKGROUND

Although [131I]meta-iodobenzylguanidine (MIBG) is currently one of the best agents available for targeted radiotherapy, its use is confined to a few neural crest derived tumours which accumulate the radiopharmaceutical via the noradrenaline transporter (NAT). To determine whether this drug could be used for the treatment of non-NAT expressing tumours following genetic manipulation, we previously showed that plasmid mediated transfection of NAT into a non-NAT expressing glioblastoma cell line, UVW, endowed the host cells with the capacity to actively accumulate [131I]MIBG. We now present data defining the conditions required for complete sterilisation of NAT transfected cells cultured as multicellular spheroids and treated with [131I]MIBG.

METHODS

NAT transfected UVW cells, grown as monolayers and spheroids, were treated with various doses of [131I]MIBG and assessed for cell kill by clonogenic survival and measurement of spheroid volume over time (growth delay). Spheroids were left intact for different time periods to assess the effect of radiation crossfire on cell death.

RESULTS AND CONCLUSIONS

Total clonogen sterilisation was observed when the cells were grown as three-dimensional spheroids and treated with 7 MBq/ml [131I]MIBG. The added benefit of radiation crossfire was demonstrated by the improvement in cell kill achieved by prolongation of the maintenance of [131I]MIBG treated spheroids in their three-dimensional form, before disaggregation and clonogenic assay. When left intact for 48 h after treatment, spheroid cure was achieved by exposure to 6 MBq/ml [131I]MIBG. These results demonstrate that the efficiency of cell kill by [131I]MIBG targeted therapy is strongly dependent on beta-particle crossfire irradiation. This gene therapy/targeted radiotherapy strategy has potential for [131I]MIBG mediated cell kill in tumours other than those derived from the neural crest.

Comparison of different methods of intracerebral administration of radioiododeoxyuridine for glioma therapy using a rat model

The Auger electron emitting agent 5-[125I]iodo-2'-deoxyuridine (i.e. [125I]IUdR) holds promise for the treatment of residual glioma after surgery because this thymidine analogue kills only proliferating cells. However, malignant cells which are not synthesizing DNA during exposure to the radiopharmaceutical will be spared. To determine whether tumour incorporation of [125I]IUdR could be enhanced by protracted administration, we used a C6 cell line, growing in the brains of Wistar rats, as a glioma model and compared three methods of intracerebral delivery of [125I]IUdR. Twenty-four hours after administration of drug, autoradiography of brain sections demonstrated nuclear uptake of the radiopharmaceutical in cells throughout tumour while normal brain cells remained free of radioactivity. The [125I]IUdR labelling indices (% +/- s.e.m.) achieved were 6.2 (0.4) by single injection, 22.5 (4.1) using a sustained release polymer implant (poly(lactide-co-glycolide)) and 34.3 (2.0) by mini-osmotic pump. These results emphasize the need for a sustained delivery system as a prerequisite for effective treatment. These findings are also encouraging for the development of a sustained release system for radiolabelled IUdR for use in the treatment of intracranial tumours, particularly in the immediate postoperative setting.

Noradrenaline transporter gene transfer for radiation cell kill by 131I meta-iodobenzylguanidine

Meta-iodobenzylguanidine conjugated to 131I-iodine is an effective agent for the targeted radiotherapy of tumors of neural crest origin which express the noradrenaline transporter (NAT). The therapeutic application of 131I MIBG is presently limited to the treatment of phaeochromocytoma, neuroblastoma, carcinoid and medullary thyroid carcinoma. To determine the feasibility of MIBG targeting for a wider range of tumor types, we employed plasmid-mediated transfer of the NAT gene into a human glioblastoma cell line (UVW) which does not express the NAT gene. This resulted in a 15-fold increase in uptake of MIBG by the host cells. A dose-dependent toxicity of 131I MIBG to the transfectants was demonstrated using three methods: (1) survival of clonogens derived from monolayer culture; (2) survival of clonogens derived from disaggregated multicellular spheroids; and (3) spheroid growth delay. 131I MIBG was twice as toxic to cells in spheroids compared with those in monolayers, consistent with a greater effect of radiation cross-fire (radiological bystander effect) from 131I beta-radiation in the three-dimensional tumor spheroids. The highest concentration of 131I MIBG tested (1 MBq/ml) was nontoxic to UVW control cells or spheroids transfected with the NAT gene in reverse orientation. These findings are encouraging for the development of NAT gene transfer-mediated 131I MIBG therapy.

Iodo-2'-deoxyuridine (IUdR) and 125IUdR loaded biodegradable microspheres for controlled delivery to the brain

The aim of this work was to develop sustained local release systems for radioiodinated iodo-2'-deoxyuridine (125IUdR) from biodegradable polymeric microspheres to facilitate the controlled delivery of 125IUdR to brain tumours. The selective uptake of IUdR into the cell nucleus results in cell disruption over the short range of the low energy Auger electrons. The biodegradable microspheres can be precisely implanted in the brain by stereotactic techniques and the IUdR within the microspheres is protected from degradation and thus a sustained source of radiolabelled IUdR is available in the vicinity of the residual tumour cells. Poly(lactic-co-glycolic acid), PLGA (85:15), microspheres containing cold IUdR and the Auger-electron emitter 125I, as 125IUdR were prepared using the O/W, O/O and W/O/W emulsion-solvent evaporation methods. The W/O/W emulsion method was most effective in achieving good drug loading with the use of bovine plasma in the internal water phase. Also effective in improving the drug loading was the use of 20% acetone in the dichloromethane and the presence of Span 40 in the organic phase. Electrolytes (NaCl and IUdR) in the external aqueous phase also improved drug loading. After an initial rapid release from the microspheres, a sustained release was observed over 15 days for the 'cold' IUdR. The sustained release portions of the release curves showed Higuchi (t1/2), diffusion controlled release kinetics. The radiolabelled IUdR microspheres showed a burst release effect of 30-40% followed by a sustained release over 35 days.