Quantitative comparison of radiolabeled antibody therapy and external beam radiotherapy in the treatment of human glioma xenografts

Extracellular vesicles role in radio(nuclide)therapy

Conventional radiation therapy can restore the ability of cells to undergo immunogenic cell death. Recent preclinical studies suggest that targeted radionuclide therapy, which delivers radiation to tumors at a continuous low dose rate, also stimulates the immune system and offers a promising approach for overcoming resistance to immune checkpoint inhibitors. In this context, we examined the growing body of preclinical and clinical findings showing that the immune system can be activated by the release of extracellular vesicles from irradiated cells, contributing to the antitumor immunity.

Immunomodulatory effects of targeted radionuclide therapy

It is now clear that conventional radiation therapy can reinstate cell death immunogenicity. Recent preclinical data indicate that targeted radionuclide therapy that irradiate tumors at continuous low dose rate also can elicit immunostimulatory effects and represents a promising strategy to circumvent immune checkpoint inhibitor resistance. In this perspective, we discuss the accumulating preclinical and clinical data suggesting that activation of the immune system through the cGAS-STING axis and the release of extracellular vesicles by irradiated cells, participate to this antitumor immunity. This should need to be considered for adapting clinical practices to state of the art of the radiobiology and to increase targeted radionuclide therapy effectiveness.

The potential and hurdles of targeted alpha therapy - clinical trials and beyond

This article presents a general discussion on what has been achieved so far and on the possible future developments of targeted alpha (α)-particle therapy (TAT). Clinical applications and potential benefits of TAT are addressed as well as the drawbacks, such as the limited availability of relevant radionuclides. Alpha-particles have a particular advantage in targeted therapy because of their high potency and specificity. These features are due to their densely ionizing track structure and short path length. The most important consequence, and the major difference compared with the more widely used β⁻-particle emitters, is that single targeted cancer cells can be killed by self-irradiation with α-particles. Several clinical trials on TAT have been reported, completed, or are on-going: four using ²¹³Bi, two with ²¹¹At, two with ²²⁵Ac, and one with ²¹²Pb/²¹²Bi. Important and conceptual proof-of-principle of the therapeutic advantages of α-particle therapy has come from clinical studies with ²²³Ra-dichloride therapy, showing clear benefits in castration-resistant prostate cancer.

Clinical radioimmunotherapy--the role of radiobiology

Conventional external-beam radiation therapy is dedicated to the treatment of localized disease, whereas radioimmunotherapy represents an innovative tool for the treatment of local or diffuse tumors. Radioimmunotherapy involves the administration of radiolabeled monoclonal antibodies that are directed specifically against tumor-associated antigens or against the tumor microenvironment. Although many tumor-associated antigens have been identified as possible targets for radioimmunotherapy of patients with hematological or solid tumors, clinical success has so far been achieved mostly with radiolabeled antibodies against CD20 ((131)I-tositumomab and (90)Y-ibritumomab tiuxetan) for the treatment of lymphoma. In this Review, we provide an update on the current challenges aimed to improve the efficacy of radioimmunotherapy and discuss the main radiobiological issues associated with clinical radioimmunotherapy.

Myeloablative radioimmunotherapy with Re-188-anti-CD66-antibody for conditioning of high-risk leukemia patients prior to stem cell transplantation: biodistribution, biokinetics and immediate toxicities

BACKGROUND

Stem cell transplantation (SCT) is potentially curative for high-risk leukemia patients. Conditioning regimens affect relapse rate and treatment-related mortality. We evaluated biodistribution, radiation absorbed organ doses and immediate toxicities of myeloablative radioimmunotherapy with marrow selective 188rhenium (188Re)-labeled anti-CD66 monoclonal antibody (mAb).

METHODS

Fifty high-risk leukemia patients were treated 14 +/- 2 days prior to SCT. Dosimetric measurements were performed at 1.5, 3, 20, 26, and 44 hours after about 1 GBq of 188Re followed by radioimmunotherapy with about 10 GBq 188Re. Standard conditioning consisted of high-dose chemotherapy and 12 Gy total-body irradiation. Forty-six patients received allogenic, and four received autologous, stem cell grafts.

RESULTS

The mean radiation absorbed doses (in Gy) were: marrow, 13.9 +/- 4.6; liver, 5.7 +/- 2.7; spleen, 22.6 +/- 25.5; kidneys, 6.8 +/- 2.6; lungs, 0.8 +/- 0.7; total body, 1.4 +/- 0.3. The tumor-to-organ-ratios were 2.4 for liver, 0.6 for the spleen, 2.0 for the kidneys and 17.8 for the lungs. Type of leukemia did not affect radiation absorbed doses of marrow, lungs, kidneys and liver. Mean marrow dose of transplanted patients in complete remission was 1.37 +/- 0.43 Gy/GBq, compared with 1.34 +/- 0.29 Gy/GBq for patients with leukemic blast marrow infiltration of 5-25%. Immediate side effects were moderate. All patients showed primary engraftment. After a median follow-up of 11.0 +/- 7.4 months 28/50 patients (56%) are in ongoing complete remission. Nine patients (5%) have relapsed, seven (4%) of them have died. Another 13 patients (7%) died of treatment-related causes.

CONCLUSIONS

Due to its biodistribution, radiation absorbed organ doses, low toxicity and clinical data, myeloablative radioimmunotherapy with 188Re-labeled anti-CD66 mAb seems to be a promising method for improving standard conditioning of high-risk leukemia patients prior to SCT.

Protracted exposure radiosensitization of experimental human malignant glioma

Clinical modulation of radiosensitivity via combined fractionated high dose rate and continuous ultra-low dose rate irradiation (ULDR) holds promise for the radiosensitization of human malignant gliomas. We measured both the in vitro and in vivo responses of a human malignant glioma cell line to combined continuous ULDR and high dose rate treatments. For in vitro ULDR treatments, U251 human malignant glioma cells were cultured in media containing tritiated water to yield a continuous dose rate of 0.03 Gy/hr. After exposures of 24, 48, or 72 hr, cells were acutely (1.1 Gy/min) irradiated, replated, and scored for colony formation. In vivo, U251 flank xenografts in nude mice had 125-iodine (125-I) seed brachytherapy at a dose rate of 0.05 Gy/hr. For whole-body continuous ULDR (0.03 Gy/hr), a 137-Cs source was mounted a fixed distance above the cages of animals bearing xenografts. After 3 days' continuous exposure, xenografts were acutely irradiated (2 Gy x 8 vs. 5 Gy x 2 daily fractions), and the regrowth delay in tumors was measured. In vitro, exposure to ULDR (0.03 Gy/hr) alone caused only modest killing and reduced the surviving fraction by approximately 0.2 logs after 72 hr exposure. The highest (10 Gy) dose of acute irradiation alone reduced survival by 1 log. However, U251 cell killing increased to 2.5 logs after combined HDR and ULDR treatments. Linear-quadratic modeling showed comparatively greater increase in the beta than the alpha coefficients of the linear-quadratic model for cell killing. In vivo, the 125-I seed brachytherapy treatments delayed tumor growth but resulted in no regression. The HDR treatments (5 Gy x 2 or 2 Gy x 8 daily fractions) caused growth delays (in days) of 17+/-2 or 16+/-2 (P=NS) days, respectively. The combined seed and 5 Gy x 2 or 2 Gy x 8 daily fractions regimen resulted in striking prolongation of regrowth delay (52.3+/-8.7 vs. 59.5+/-7.7 days) (P < 0.001 vs. HDR treatments alone). External ULDR alone caused no regression and minimal growth delay. Combined continuous external ULDR and the 5 Gy x 2 vs. 2 Gy x 8 daily fraction regimens resulted in prolongation of growth delay (33+/-0.9 (P=0.01 vs. 5 Gy x 2 daily fractions alone) vs. 35+/-0.7 (P=0.049 vs. 2 Gy x 8 daily fractions alone). We conclude that continuous ULDR increases the effect of HDR treatments of experimental malignant glioma. This increased effect may prove clinically important in the treatment of human malignant brain tumors.

Synthetic, implantable polymers for local delivery of IUdR to experimental human malignant glioma

PURPOSE

Recently, polymeric controlled delivery of chemotherapy has been shown to improve survival of patients with malignant glioma. We evaluated whether we could similarly deliver halogenated pyrimidines to experimental intracranial human malignant glioma. To address this issue we studied the in vitro release from polymers and the in vivo drug delivery of IUdR to experimental human U251 glioblastoma xenografts.

METHODS AND MATERIALS

In vitro: To measure release, increasing (10%, 30%, 50%) proportions of IUdR in synthetic [(poly(bis(p-carboxyphenoxy)-propane) (PCPP):sebacic acid (SA) polymer discs were serially incubated in buffered saline and the supernatant fractions were assayed. In vivo: To compare local versus systemic delivery, mice bearing flank xenografts had intratumoral or contralateral flank IUdR polymer (50% loading) treatments. Mice bearing intracranial (i.c.) xenografts had i.c. versus flank IUdR polymer treatments. Four or 8 days after implantation of polymers, mice were sacrificed and the percentage tumor cells that were labeled with IUdR was measured using quantitative microscopic immunohistochemistry.

RESULTS

In vitro: Increasing percentage loadings of IUdR resulted in higher percentages of release: 43.7 + 0.1, 70.0 + 0.2, and 90.2 + 0.2 (p < 0.001 ANOVA) for the 10%, 30%, and 50% loadings, respectively. In vivo: For the flank tumors, both the ipsilateral and contralateral IUdR polymers resulted in similarly high percentages labeling of the tumors versus time. For the ipsilateral IUdR polymers, the percentage of tumor cellular labeling after 4 days versus 8 days was 45.8 +/- 7.0 versus 40.6 +/- 3.9 (p = NS). For the contralateral polymer implants, the percentage of tumor cellular labeling were 43.9 +/- 10.1 versus 35.9 +/- 5.2 (p = NS) measured 4 days versus 8 days after implantation. For the i.c. tumors treated with extracranial IUdR polymers, the percentage of tumor cellular labeling was low: 13.9 +/- 8.8 and 11.2 +/- 5.7 measured 4 and 8 days after implantation. For the i.c. tumors having the i.c. IUdR polymers, however, the percentage labeling was comparatively much higher: 34.3 +/- 4.9 and 35.3 +/- 4.0 on days 4 and 8, respectively. For the i.c. tumors, examination of the percentage cellular labeling versus distance from the implanted IUdR polymer showed that labeling was highest closest to the polymer disc.

CONCLUSION

Synthetic, implantable biodegradable polymers provide the local, controlled release of IUdR and result in the high, local delivery of IUdR to experimental intracranial human malignant glioma. This technique holds promise for the local delivery of IUdR for radiosensitization of human brain tumors.

Effect of interstitial and/or systemic delivery of tirapazamine on the radiosensitivity of human glioblastoma multiforme in nude mice

The purpose of this study was to investigate the feasibility and the efficacy of administering tirapazamine by a slow-releasing polymer disc that was implanted interstitially into a U251 (human glioblastoma multiforme) tumor grown in nude mice. Tumor-bearing animals, with a tumor nodule 0.8 cm3 in size, were distributed to groups receiving combinations of empty or drug-containing polymer implants in the tumor or contralateral leg, intraperitoneal (i.p.) drug, and/or irradiation. The drug (i.p.) alone (14 mg/kg x6) or in combination with tumor drug implant (2 mg) did not significantly increase the tumor volume doubling time compared to that of control animals. Given with 12 Gy of irradiation in twice a day 2-Gy fractions, combined i.p. drug and tumor drug implant significantly delayed tumor growth compared to irradiation alone, which was not achieved with either drug treatment alone added to irradiation. Toxicity, as manifested by transient weight loss, was primarily seen in animals receiving radiation and i.p. tirapazamine. These results indicated that a slow-releasing tirapazamine disc can be produced and the addition of an interstitially implanted tirapazamine disc further increased the effectiveness of i.p. tirapazamine.

Implantable biodegradable polymers for IUdR radiosensitization of experimental human malignant glioma

PURPOSE

The potential of halogenated pyrimidines for the radiosensitization of human malignant gliomas remains unrealized. To assess the role of local delivery for radiosensitization, we tested a synthetic, implantable biodegradable polymer for the controlled release of 5-iodo-2'-deoxyuridine (IUdR) both in vitro and in vivo and the resultant radiosensitization of human malignant glioma xenografts in vivo.

MATERIALS AND METHODS

In vitro: To measure release, increasing (10%, 30%, 50%) proportions (weight/weight) of IUdR in the polyanhydride [(poly(bis(p-carboxyphenoxy)-propane) (PCPP): sebacic acid (SA) (PCPP : SA ratio 20:80)] polymer discs were incubated (1 ml phosphate-buffered saline, 37 degrees C). The supernatant fractions were serially assayed using high performance liquid chromatography. To measure modulation of release, polymer discs were co-loaded with 20 microCi 5-125-iodo-2'-deoxyuridine (125-IUdR) and increasing (10%, 30%, or 50%) proportions of D-glucose. To test radiosensitization, cells (U251 human malignant glioma) were sequentially exposed to increasing (0 or 10 microM) concentrations of IUdR and increasing (0, 2.5, 5.0, or 10 Gy) doses of acute radiation. In vivo. To measure release, PCPP : SA polymer discs having 200 microCi 125-IUdR were surgically placed in U251 xenografts (0.1-0.2 cc) growing in the flanks of nude mice. The flanks were reproducibly positioned over a collimated scintillation detector and counted. To measure radiosensitization, PCPP : SA polymer discs having 0% (empty) or 50% IUdR were placed in the tumor or contralateral flank. After five days, the tumors were acutely irradiated (500 cGy x 2 daily fractions).

RESULTS

In vitro: Intact IUdR was released from the PCPP : SA polymer discs in proportion to the percentage loading. After 4 days the cumulative percentages of loaded IUdR that were released were 43.7 +/- 0.1, 70.0 +/- 0.2, and 90.2 +/- 0.2 (p < 0.001 ANOVA) for the 10, 30, and 50% loadings. With 0, 10, 30, or 50% D-glucose co-loading, the cumulative release of 125-IUdR from PCPP : SA polymers was 21, 70, 92, or 97% (p < 0.001), respectively, measured 26 days after incubation. IUdR radiosensitized U251 cells in vitro. Cell survival (log10) was -2.02 +/- 0.02 and -3.68 +/- 0.11 (p < 0.001) after the 10 Gy treatment and no (control) or 10 microM IUdR exposures, respectively. In vivo: 125-IUdR Release: The average counts (log10 cpm +/- SEM) (hours after implant) were 5.2 +/- 0.05 (0.5), 4.3 +/- 0.07 (17), 3.9 +/- 0.08 (64), and 2.8 +/- 0.06 (284). Radiosensitization: After intratumoral implantation of empty polymer or intratumoral 50% IUdR polymer, or implantation of 50% IUdR polymers contralateral to tumors the average growth delays of tumors to 4 times the initial volumes were 15.4 +/- 1.8, 20.1 + 0.1, and 20.3 + 3.6 (mean + SEM) days, respectively (p = 0.488 one-way ANOVA). After empty polymer and radiation treatments, no tumors regressed and the growth delay was 31.1 + 2.1 (p = 0.046 vs. empty polymer alone) days. After implantation of 50% IUdR polymers either contralateral to the tumors or inside the tumors, followed by radiation, tumors regressed; growth delays to return to the initial average volumes of 14.0 + 3.6 or 24.2 + 0.2 (p < 0.01) days, respectively.

CONCLUSIONS

Synthetic, implantable biodegradable polymers hold promise for the controlled release and local delivery of IUdR for radiosensitization of gliomas.

Prediction of tumor response to experimental radioimmunotherapy with 90Y in nude mice

PURPOSE

To identify those factors that predict variability in tumor response to 90Y-radioimmunotherapy based on measurement of incorporated activity and physical dimensions of individual tumors and to apply the concept of effective dose to radioimmunotherapy.

METHODS AND MATERIALS

Human colon carcinoma xenografts growing in nude mice were treated with anti-CEA antibodies labeled with 90Y directly or through a bispecific antibody/labeled hapten system. Tumor response was measured as the delay in growth to eight times the treatment volume. Noninvasive activity (based on bremsstrahlung radiation) and dimension measurements were made in these animals at several times after label injection. The following parameters were compared for their ability to predict individual tumor response: (a) injected activity, (b) injected activity times a factor based on average uptake as a function of volume, (c) in vivo activity per volume measured in each animal at a single time, (d) the integral over time of in vivo activity per volume in each animal, and (e) the minimum dose for each animal in a uniformly active ellipsoid whose total activity and dimensions varied over time the same as the tumor.

RESULTS AND CONCLUSION

After correcting for differences in injected activity, two parameters account for much of the variability in tumor response. One of these is the general trend of larger tumors to take up less activity per volume. Additional variability can be accounted for by the in vivo activity per volume measurements. The minimum dose as introduced here is likely to be useful in estimating the biologically effective dose delivered by each treatment.

Direct measurement of intratumor dose-rate distributions in experimental xenografts treated with 90Y-labeled radioimmunotherapy

PURPOSE

To measure, quantify, and evaluate the planar dose-rate distribution for human tumor xenografts implanted into mice that are treated with 90Y-labeled monoclonal antibodies or bispecific antibodies and 90Y-labeled haptens.

METHODS AND MATERIALS

Twenty-five LS174T human colon carcinoma tumors grown subcutaneously in nude mice were treated with 90Y by either directly labeled ZCE025 or bispecific ECA001-DBX antibody systems. A simple, quick technique using GAF radiochromic medium determined the dose-rate distribution in a plane passing through the tumor center. The dose-rate distribution is generated from exposure to activity situated in one-half of the tumor (0.045 to 0.83 g).

RESULTS

Planar dose-rate distributions were obtained from the tumor xenografts. Planar dose-rate histograms were computed along with the coefficients of variance and skewness of the distributions. The observed dose-rate distributions were quantitatively compared to those calculated for a uniformly distributed activity in a half-ellipsoid of the same volume and approximate shape as the tumor half. The observed dose-rate distributions were usually broader with a more positive coefficient of skewness than the dose-rate distributions calculated from the uniformly active half-ellipsoids. For 90Y, tumor shape plays an important role in determining the minimum tumor dose. For these tumors, the tumor minimum dose-rate is always observed along the edge, usually where the edge curvature is most convex. Larger tumors tended to have broader dose-rate distributions and more positive coefficients of skewness. Exceptions to this trend were associated with dose-rate maxima displaced from the central regions due to activity heterogeneity or tumor size greatly exceeding the range of emission. Calculations for dose rate from the conventional Medical Internal Radiation Dose (MIRD) formulation exceeded the average and minimum dose rate derived from radiochromic media. The coefficient of skewness became more positive for increasing time between injection and tumor excision, consistent with the activity evolving into a more uniform activity distribution.

CONCLUSION

Using radiochromic media to measure the spatial dose-rate distribution is a valuable method for comparing the dose-rate heterogeneity among experimental tumor xenografts in animals treated with radiolabeled antibodies. Tumor size (relative to the particle range) and changes in activity distribution radiolabeled antibodies. Tumor size (relative to the particle range) and changes in activity distribution affect the dose-rate distribution that are reflected by changes in the coefficients of skewness and variation of the dose-rate area histogram. The increase in coefficients of variation and skewness with tumor size and time results from the size of the 90Y beta particle penetration range that either exceeds or is comparable to the tumor dimensions. The minimum dose rate is more dependent, relative to the average and the maximum dose rates, on the curvature of the tumor surface.

Use of bremsstrahlung radiation to monitor Y-90 tumor and whole body activities during experimental radioimmunotherapy in mice

BACKGROUND

Large differences in uptake between tumors, even for the same size, frequently observed in clinical and experimental radioimmunotherapy (RAIT), make monitoring of uptake in individual tumors imperative in comparing protocols. 90Y, widely-used for RAIT, emits no gamma radiation and absorption of the beta particle in tissue makes its detection unsuitable for in vivo monitoring. We tested whether bremsstrahlung radiation, produced when betas are decelerated by nuclei, could be used to monitor tumor uptake.

METHODS

Subcutaneous human LS174T colon carcinoma tumors were grown in the upper thigh of nude mice and labeled antibody injected intracardially. With the tumor placed in the 2 cm-diameter aperture in a lead collimator, photons with energies from 100 to 200 keV transmitted through plastic, which absorbed the beta particles, were counted to maximize shielding from the rest of the body. The contribution of the normal tissues was subtracted by counting the non-tumor-bearing leg in the same position. Excretion was calculated from whole body activity determined by removing the collimator, placing the mouse in a syringe surrounded by tissue-equivalent material 10 cm from the detector, and counting photons between 200 and 740 keV to minimize the effect of tissue attenuation.

RESULTS

For tumors larger than 0.14 gm, a good correlation was obtained between the in vivo bremsstrahlung measurements and the measurements on excised tumors in a calibrated well counter. Similar excretion rates observed in all the animals suggested that the whole body counting was accurate.

CONCLUSIONS

Bremstrahlung detection appears feasible and reliable for monitoring both tumor and whole body activities.

Overview of animal studies comparing radioimmunotherapy with dose equivalent external beam irradiation

As the field of radioimmunotherapy (RIT) continues to develop and looks increasingly promising, there is growing interest in the radiobiology of RIT. Recently, several investigators have conducted studies in animal models comparing the relative efficacy of RIT with dose equivalent external beam irradiation. Although these studies are the first of many to follow, the results are provocative and several patterns are suggested by the available data. The results of the studies are summarized and compared, and preliminary hypotheses that might explain the reported observations are discussed. In summary, results from studies comparing the efficacy of RIT with external beam irradiation have been variable and may be indicative of different underlying mechanisms. While the particular experimental model, design and methodology used to compare the efficacy of RIT with external beam irradiation are probably important influences upon subsequent observations, it appears that for a given tumor type, the size of the survival curve shoulder or alpha/beta ratio, and tumor doubling time are important determinants of the magnitude of the dose rate effect. When this effect is minimal, it is possible that other factors such as reoxygenation, the arrest of cells in G2, and selective targeting of tumor by radiolabelled antibody may explain, in part, the increased efficacy of RIT compared with external beam irradiation that has been observed in some systems.