Status of radioimmunotherapy in the new millennium

Risks and untoward toxicities of antibody-based immunoconjugates

Antibody-based immunoconjugates are specifically targeted monoclonal antibodies that deliver a cytotoxic payload to their target. The cytotoxic agents can be highly potent drugs, radionuclides or toxins. Such molecules, referred to as antibody-drug conjugates, radioimmunoconjugates and immunotoxins, respectively, represent a promising approach for enhancing the efficacy of unconjugated (naked) antibodies for improved therapeutic results. Though tremendous progress has been achieved over the last few decades, the safety of these molecules still remains a matter of concern and a careful design is required for achieving a relatively safe toxicity profile along with therapeutic effectiveness. This review focuses on the toxicities arising from the use of these potent agents.

In vivo quantitation of intratumoral radioisotope uptake using micro-single photon emission computed tomography/computed tomography

PURPOSE

This study was undertaken to determine the ability of micro-single photon emission computed tomography (micro-SPECT)/computed tomography (CT) to accurately quantitate intratumoral radioisotope uptake in vivo and to compare these measurements with planar imaging and micro-SPECT imaging alone.

PROCEDURES

Human pancreatic cancer xenografts were established in 10 mice. Intratumoral radioisotope uptake was achieved via intratumoral injection of an attenuated measles virus vector expressing the NIS gene (MV-NIS). On various days after MV-NIS injection, (123)I planar and micro-SPECT/CT imaging was performed. Tumor activity was determined by dose calibrator measurements and region-of-interest (ROI) image analysis. Agreement and reproducibility of tumor activity measurements were assessed by Bland-Altman plots and Lin's concordance correlation coefficient (CCC).

RESULTS

Intratumoral radioisotope uptake was detected in all mice. Scatterplots demonstrate strong agreement (CCC = 0.93) between micro-SPECT/CT ROI image analysis and dose calibrator tumor activity measurements. The differences between dose calibrator activity measurements and those obtained with ROI image analysis of micro-SPECT alone and planar imaging are less accurate and more variable (CCC = 0.84 and 0.78, respectively).

CONCLUSIONS

Micro-SPECT/CT can be used to accurately quantify intratumoral radioisotope uptake in vivo and is more reliable than planar or micro-SPECT imaging alone.

Radioimmunotherapy of solid tumors by targeting extra domain B fibronectin: identification of the best-suited radioimmunoconjugate

PURPOSE

The expression of extra domain B (ED-B) fibronectin is always associated with angiogenic processes and can be exclusively observed in tissues undergoing growth and/or extensive remodeling. Due to this selective expression, ED-B fibronectin is an interesting target for radioimmunotherapy of malignant diseases. The aim of this study was to identify the most appropriate ED-B-targeting radioimmunoconjugate for the therapy of solid tumors.

EXPERIMENTAL DESIGN

Three ED-B fibronectin-binding human antibody formats of L19 were investigated: dimeric single-chain Fv (approximately 50 kDa), "small immunoprotein" (SIP, approximately 80 kDa), and immunoglobulin G1 (IgG1, approximately 150 kDa). These L19 derivatives were either labeled with I-125 or with In-111 (using MX-diethylenetriaminepentaacetic acid, MX-DTPA). Pharmacokinetics and tumor accumulation of the radiolabeled immunoconjugates were investigated in F9 (murine teratocarcinoma) tumor-bearing mice. Subsequently, dosimetry for the corresponding therapeutic isotopes I-13-1 and Y-90 was done. After testing the myelotoxicity of I-131-L19-SIP and I-131-L19-IgG1 in non-tumor-bearing mice, the therapeutic efficacy of these iodinated antibody formats was finally investigated in F9 tumor-bearing mice.

RESULTS

The most favorable therapeutic index was found for I-131-L19-SIP followed by I-131-L19-IgG1. The therapeutic index of all In-111-labeled derivatives was significantly inferior. Considering the bone marrow as the dose-limiting organ, it was calculated that activities of 74 MBq I-131-L19-SIP and 25 MBq I-131-L19-IgG1 could be injected per mouse without causing severe myelotoxicity. The best therapeutic efficacy was observed using I-131-L19-SIP, resulting in significant tumor growth delay and prolonged survival after a single injection.

CONCLUSION

Compared with other L19-based radioimmunoconjugates, I-131-L19-SIP is characterized by superior antitumor efficacy and toxicity profile in the F9 teratocarcinoma animal model. These results indicate that ED-B fibronectin-targeted radioimmunotherapy using I-131-L19-SIP has potential to be applied to treatment of solid cancers.

Penetratin Improves Tumor Retention of Single-Chain Antibodies: A Novel Step toward Optimization of Radioimmunotherapy of Solid Tumors

Single-chain Fv (scFv) antibody fragments exhibit improved pharmacokinetics and biodistribution compared with intact IgG. The tumor uptake of scFvs is rapid, and the serum half-life is shorter than IgG. However, scFvs exhibit lower net dose deposition in the tumor due to a shorter residence time that limits their use in radioimmunotherapy. To improve the tumor uptake and retention of scFvs, we investigated the utility of cell-penetrating peptides, penetratin and transactivator of transcription (TAT). Biodistribution studies were done in LS174T tumor-bearing mice with divalent scFv derived from anti-tumor-associated glycoprotein 72 monoclonal antibody (mAb) CC49. Penetratin increased the tumor retention of scFvs without affecting the peak dose accumulation. The percentage of doses retained in tumors at 24 hours post-administration with a control (no peptide), penetratin, and TAT were 27.25%, 79.84%, and 48.55%, respectively, of that accumulated at 8 hours postinjection. The tumor-to-blood ratios at 24 hours postadministration were 7.14, 19.53, and 16.48 with control, penetratin, and TAT treatment, respectively, whereas the pharmacokinetics were unaltered. Coinjection with TAT, however, resulted in increased uptake of the radioconjugate by the lungs. Autoradiography of the excised tumors indicated a more homogenous distribution of the radiolabeled scFv with both penetratin and TAT in comparison with the control treatment. Real-time whole-body imaging of the live animals confirmed improved tumor localization with penetratin without any increase in the uptake by normal tissues. In conclusion, a significant improvement in the tumor retention of sc(Fv)2 was achieved by administration of penetratin. Therefore, the combination of penetratin and scFvs has the potential of improving the utility of mAb-based radiopharmaceuticals.

Engineering and characterization of a divalent single-chain Fv angiotensin II fusion construct of the monoclonal antibody CC49

For the therapy of solid tumors, co-administration of angiotensin II (AngII) results in an increased uptake of drugs into the tumor interstitium. We have engineered a dimeric sc(Fv)(2)-AngII fusion construct that combines the superior kinetics of covalent dimeric scFvs [sc(Fv)(2)], recognizing the pancarcinoma tumor-associated antigen 72 (TAG-72), with the advantageous intrinsic activity of AngII. The binding characteristics of the fusion construct were unaltered by the addition of the AngII sequence [affinity constant K(A) 1.18 x 10(7) and 8.42 x 10(6) M(-1) for sc(Fv)(2) and sc(Fv)(2)-AngII, respectively]. The binding of the fusion construct to the angiotensin receptor (AT(1)) was similar to AngII, and the arterial contraction was 16 +/- 1% of the response observed with norepinephrine. In animal studies, the radiolabeled sc(Fv)(2)-AngII construct exhibited similar uptake and a more homogeneous distribution within the tumor as compared to sc(Fv)(2).

Locoreginal radioimmunotherapy with Re-labeled monoclonal antibody in treating small peritoneal carcinomatosis of colon cancer in mice in comparison with I-counterpart

The efficacy of locoregional radioimmunotherapy (RIT) in treating peritoneal tumors of colon cancer of <2 mm in diameter was examined at maximum tolerated doses, focusing the comparison between 186Re and 131I labeled to an anti-colorectal cancer IgG1. Estimated radiation doses to tumors were considerably higher with 186Re-RIT than with 131I-RIT. The advantage of 186Re-RIT decreased with decreasing tumor size, but 186Re-RIT delivered 1.6-times higher radiation to tumors of 1 mm. Consequently, 186Re-RIT attained better survival of mice than 131I-RIT or chemotherapy with 5-fluorouracil did. Therefore, locoregional 186Re-RIT may be an option in an adjuvant setting of colon cancer with high risk of peritoneal dissemination.

Optimization of radioimmunotherapy interactions with hyperthermia

The efficacy of radioimmunotherapy (RIT) employing radiolabelled monoclonal antibodies (MAb) is currently limited in most solid tumours. The combination of local hyperthermia (HT) with RIT has the potential to enhance tumour targeting of MAb; moreover, this approach may add an antitumour effect to radioresistant hypoxic and S-phase cells and may inhibit the cells from repairing sublethal damage or potentially lethal damage caused by ionizing radiation. There are distinct types of protocols in this combination. Hyperthermic temperature and timing relative to RIT administration appear to affect the efficacy of the combination therapy. Responses to heating at any particular condition are not always the same among different tumour types. There are many papers describing influence of HT on the biodistribution of radiolabelled MAb, but only limited information is currently available on 'therapeutic' outcomes regarding the dependency of combination protocols. A previous study suggested that the best therapeutic improvement would be achieved when HT was combined immediately after the administration of MAb, which significantly increases the radiation absorbed dose to tumours and produces a uniform intratumoural dose distribution. Further therapeutic investigation should be required to reach the optimal protocol of combining these two modalities.

Early effects of low dose-rate radiation on cultured tumor cells

Low-dose hyperradiosensitivity (HRS) has been found for several cell types after exposure to low doses, < 0.5 Gy, of high dose-rate (typically 50-150 Gy/h) low-LET radiation. HRS precedes the occurrence of a relative resistance for doses above 0.5-1 Gy. A critical question is whether HRS is of importance in radionuclide therapy where the dose-rate is low but the total dose might be high. An indication that cells exposed to low dose-rate can be kept hyperradiosensitive has recently been published. We have in the present study applied cells without (glioma U373MG) and with (glioma U118MG and colon carcinoma HT29) HRS and studied early effects, up to one week, during low dose-rate (LDR), 0.05-0.09 Gy/hours, exposure (total dose after one week: 11.8 +/- 1.5 Gy). The cells were grown on thin foils above a (32)P source placed in a cell culture chamber. Cell number reductions, cell-cycle disturbances, and changed numbers of apoptotic cells were analyzed after continuous LDR exposures. There seemed to be no relation with HRS when the cell number reduction was considered. The U373MG cells, lacking HRS, had the strongest cell number reduction due to a combination of a G(2) block and increased apoptosis. The U118MG and HT29 cells, both having HRS, had surprisingly low cell number reductions. U118MG had only a G(2) block but no increase in apoptosis. HT29 had both a G(2) block and an increase in apoptosis but the apoptosis change was somewhat smaller than for U373MG. Thus, there seemed to be no obvious relation between HRS and early cellular effects when the cells were analyzed after continuous LDR exposure.

Imaging and therapy of tumors induced to express somatostatin receptor by gene transfer using radiolabeled peptides and single chain antibody constructs

The fields of radioimmunodetection and radioimmunotherapy began with an initial paradigm that a targeting molecule (eg, antibody) carrying a radioisotope had the potential of selectively imaging and delivering a therapeutic dose of radiation to tumor sites. A second paradigm was developed in which injection of the targeting molecule was separated from injection of a short-lived radioisotope-labeled ligand (so-called "pretargeting strategy"). This strategy has improved radioisotope delivery to tumors in animal models, enhanced radioimmune imaging in man, and therapeutic trials are in an early phase. We proposed a third paradigm to achieve radioisotopic localization at tumor sites by inducing tumor cells to synthesize a membrane expressed receptor with a high affinity for infused radiolabeled ligands. The use of gene transfer technology to induce expression of high affinity membrane receptors can enhance the specificity of radioligand localization, while the use of radioisotopes with the ability to deliver radiation damage across several cell diameters will compensate for less than perfect transduction efficiency. This approach was termed "Genetic Radioisotope Targeting Strategy." Using this strategy, induction of high levels of gastrin releasing peptide receptor or human somatostatin receptor subtype 2 expression and selective tumor uptake of radiolabeled peptides was achieved. The advantages of the genetic transduction approach are (1) constitutive expression of a tumor-associated antigen/receptor is not required; (2) tumor cells are altered to express a new target receptor or increased quantities of an existing receptor at levels that may significantly improve tumor targeting of radiolabeled ligands compared with normal tissues; (3) gene transfer can be achieved by intratumoral or regional injection of gene vectors; (4) it is feasible to target adenovirus vectors to receptors overexpressed on tumor cells by modifying adenoviral tropism (binding) so that the virus will be targeted specifically to the desired tumor; and (5) it is possible to coexpress the receptor gene and a therapeutic gene, such as cytosine deaminase, for molecular prodrug therapy to produce an enhanced therapeutic effect.

Intraperitoneal radioimmunotherapy in treating peritoneal carcinomatosis of colon cancer in mice compared with systemic radioimmunotherapy

Peritoneal spread is one of major causes of mortality in colorectal cancer patients. In the current investigation, the efficacy of radioimmunotherapy (RIT) with i.p. administration of an anti-colorectal cancer IgG1, 131I-A7, was compared to that with i.v. administration in BALB/c female mice bearing peritoneal nodules of LS180 human colon cancer cells, at the same toxicity level. Distribution of either i.p. or i.v. administered 131I-A7 and i.p. administered irrelevant 131I-HPMS-1 was assessed. Based on the results of toxicity determination at increments of 2 MBq and estimated dosimetry, an i.p. dose of 11 MBq and an i.v. dose of 9 MBq were chosen for treatment. Mice were monitored for long-term survival: untreated mice (n = 11), mice undergoing i.p. RIT with 131I-A7 (n = 11), mice undergoing i.v. RIT with 131I-A7 (n = 11) and mice undergoing non-specific i.p. RIT with 131I-HPMS-1 (n = 5). Intraperitoneal injection of 131I-A7 produced faster and greater tumor accumulation than i.v. injection: 34.2 +/- 16.5% of the injected dose per g (% ID/g) and 11.1 +/- 3.6% ID/g at 2 h, respectively (P < 0.0001). Consequently, cumulative radioactivity in tumors was 1.73-fold higher with i.p. injection. 131I-HPMS-1 did not show specific accumulation. Non-specific RIT with 131I-HPMS-1 (mean survival, 26.0 +/- 2.5 days) did not affect the survival as compared to no treatment (26.7 +/- 1.9 days). Intravenous RIT with 131I-A7 prolonged the survival of mice to 32.8 +/- 1.8 days (P < 0.01). Intraperitoneal RIT with 131I-A7 improved the survival more significantly and attained cure in 2 of 11 mice (P < 0.05 vs. i.v. RIT). In conclusion, i.p. RIT is more beneficial in treating peritoneal carcinomatosis of colon cancer than i.v. RIT in a murine model.

Tumour therapy with radionuclides: assessment of progress and problems

Radionuclide therapy is a promising modality for treatment of tumours of haematopoietic origin while the success for treatment of solid tumours so far has been limited. The authors consider radionuclide therapy mainly as a method to eradicate disseminated tumour cells and small metastases while bulky tumours and large metastases have to be treated surgically or by external radiation therapy. The promising therapeutic results for haematological tumours give hope that radionuclide therapy will have a breakthrough also for treatment of disseminated cells from solid tumours. New knowledge related to this is continuously emerging since new molecular target structures are being characterised and the knowledge on pharmacokinetics and cellular processing of different types of targeting agents increases. There is also improved understanding of the factors of importance for the choice of appropriate radionuclides with respect to their decay properties and the therapeutic applications. Furthermore, new methods to modify the uptake of radionuclides in tumour cells and normal tissues are emerging. However, we still need improvements regarding dosimetry and treatment planning as well as an increased knowledge about the tolerance doses for normal tissues and the radiobiological effects on tumour cells. This is especially important in targeted radionuclide therapy where the dose rates often are lower than 1Gy/h.

Radioimmunotherapy with 186Re-labeled monoclonal antibody to treat liver metastases of colon cancer cells in nude mice

The efficacy of radioimmunotherapy (RIT) in the treatment of minimal disease has been previously shown, despite the limitation of beta-emitters suggested by a mathematical model. In the present study, the efficacy of RIT with an anti-colorectal cancer IgG1 A7 conjugated with 186Re was examined in a liver metastasis model established by intrasplenic inoculation of human colon cancer cells. In this model, small metastases of less than 1 mm in diameter can be observed 1 week after cell inoculation. Metastases attain a diameter of several millimeters at 2 weeks. 186Re-A7 accumulated exclusively in metastases, displaying a value of 24.1 +/- 8.7% ID/g 2 days after the injection. 186Re-A7 accumulation in liver metastases increased with decreasing size. RIT with 7 MBq 186Re-A7 at 2 weeks significantly suppressed the growth of metastases; weight of metastases 4 weeks after cell inoculation was 5.96 +/- 0.87 g in nontreated control mice and 1.25 +/- 0.75 g in mice receiving 186Re-A7 RIT (p < 0.0001). RIT at 1 week more effectively inhibited metastatic growth to 0.08 +/- 0.05 g (p < 0.002 vs. RIT at 2 weeks). RIT with a class-matched irrelevant MAb 186Re-HPMS-1 at 1 week after cell inoculation somewhat suppressed metastatic growth, 3.39 +/- 0.25 g at 4 weeks, as compared with the control; however, 186Re-HPMS-1 RIT was far less effective than 186Re-A7 RIT (p < 0.0001). These results support the use of RIT with 186Re-MAb in an adjuvant setting in cases involving minimal disease. Factors such as higher and homogeneous MAb accumulation in small nodules, better perfusion, and subsequent better oxygenation likely compensate for the loss of beta radiation outside small metastases.