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Clinical Cancer Research Vol. 11, 5920-5927, August 15, 2005
© 2005 American Association for Cancer Research

Cancer Therapy: Clinical

High-Dose Radioimmunotherapy Combined with Fixed, Low-Dose Paclitaxel in Metastatic Prostate and Breast Cancer by Using a MUC-1 Monoclonal Antibody, m170, Linked to Indium-111/Yttrium-90 via a Cathepsin Cleavable Linker with Cyclosporine to Prevent Human Anti-mouse Antibody

Carol M. Richman1, Sally J. DeNardo1, Robert T. O'Donnell1, Aina Yuan1, Sui Shen2, Desiree S. Goldstein1, Joseph M. Tuscano1, Ted Wun1, Helen K. Chew1, Primo N. Lara1, David L. Kukis1, Arutselvan Natarajan1, Claude F. Meares1, Kathleen R. Lamborn3 and Gerald L. DeNardo1

Authors' Affiliations: 1 University of California-Davis, Sacramento, California; 2 University of Alabama, Birmingham, Alabama; and 3 University of California-San Francisco, San Francisco, California

Requests for reprints: Carol M. Richman, University of California-Davis, 4501 X Street, Suite 3016, Sacramento, CA 95817. Phone: 916-734-3771; Fax: 916-734-7946; E-mail: carol.richman{at}ucdmc.ucdavis.edu.


   Abstract
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Materials and Methods
 Results
 Discussion
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Purpose: Although radioimmunotherapy alone is effective in lymphoma, its application to solid tumors will likely require a combined modality approach. In these phase I studies, paclitaxel was combined with radioimmunotherapy in patients with metastatic hormone-refractory prostate cancer or advanced breast cancer.

Experimental Design: Patients were imaged with indium-111 (111In)-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-peptide-m170. One week later, yttrium-90 (90Y)-m170 was infused (12 mCi/m2 for prostate cancer and 22 mCi/m2 for breast cancer). Initial cohorts received radioimmunotherapy alone. Subsequent cohorts received radioimmunotherapy followed 48 hours later by paclitaxel (75 mg/m2). Cyclosporine was given to prevent development of human anti-mouse antibody.

Results: Bone and soft tissue metastases were targeted by 111In-m170 in 15 of the 16 patients imaged. Three prostate cancer patients treated with radioimmunotherapy alone had no grade 3 or 4 toxicity. With radioimmunotherapy and paclitaxel, two of three prostate cancer patients developed transient grade 4 neutropenia. Four breast cancer patients treated with radioimmunotherapy alone had grade 3 or 4 myelosuppression. With radioimmunotherapy and paclitaxel, both breast cancer patients developed grade 4 neutropenia. Three breast cancer patients required infusion of previously harvested peripheral blood stem cells because of neutropenic fever or bleeding. One patient in this trial developed human anti-mouse antibody in contrast to 12 of 17 patients in a prior trial using m170-radioimmunotherapy without cyclosporine.

Conclusions: 111In/90Y-m170 targets prostate and breast cancer and can be combined with paclitaxel with toxicity limited to marrow suppression at the dose levels above. The maximum tolerated dose of radioimmunotherapy and fixed-dose paclitaxel with peripheral blood stem cell support has not been reached. Cyclosporine is effective in preventing human anti-mouse antibody, suggesting the feasibility of multidose, "fractionated" therapy that could enhance clinical response.

Radioimmunotherapy using systemically administered monoclonal antibodies linked to radioisotopes has proven effective in clinical trials in hematologic malignancies (1–8). Commercially available agents are now in routine use for treatment of non-Hodgkin's lymphoma (9–12). However, the efficacy of radioimmunotherapy in solid tumors has been limited (13–21). Given the exquisite radiosensitivity of hematologic tumors, it is likely that the treatment of more resistant solid tumors will require the administration of higher doses of radiation and the addition of radiation-enhancing approaches, as are often employed with external beam irradiation (22–27). In these two parallel phase I clinical trials in metastatic prostate and breast cancer, we employed strategies to (a) enhance the antitumor effect of radioimmunotherapy by the addition of radiosensitizing chemotherapy, (b) decrease the potential toxicity of radioimmunotherapy with the use of a new cleavable linker that facilitates elimination of the radiometal from the liver, and (c) maintain the potential for administration of "fractionated" multidose radioimmunotherapy by using cyclosporine to prevent the development of antibody to the murine, tumor-targeting antibody.


   Materials and Methods
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 Materials and Methods
Results
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Patients. Patients with hormone-refractory metastatic prostate cancer and patients with metastatic breast cancer who failed at least one prior chemotherapy for metastatic disease were eligible for imaging/therapy with indium-111 (111In)/yttrium-90 (90Y)-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA)-peptide-m170 (one trial for prostate and one for breast cancer) if they had absolute neutrophil counts >1,500/µL, platelet counts >100,000/µL, serum creatinine and bilirubin <1.5 mg/dL, pulmonary function tests at least 65% of predicted, a cardiac left ventricular ejection fraction of at least 50%, Karnofsky performance status of at least 70%, and documentation of reactivity of their tumor with the MUC-1 antibody, m170, by 111In-DOTA-peptide-m170 imaging showing uptake in areas of known tumor (both prostate and breast cancer patients) and by immunohistochemical staining of fresh or frozen tumor tissue (breast cancer patients). Patients were free of other serious medical illnesses, had no active central nervous system disease, had no chemotherapy or radiation for at least 4 weeks, and had a negative serum human anti-mouse antibody (HAMA; ref. 28) before protocol entry. Computed tomography and bone scans as well as serum tumor markers (prostate-specific antigen, CA 15-3, and carcinoembryonic antigen) were obtained before protocol entry and thereafter to document disease location and to assess response. Serial clinical and laboratory evaluations, including pulmonary function tests, cardiac function studies, and hematology and chemistry studies, were done to assess toxicity. All patients who participated in these studies signed written informed consent in accordance with the institutional review board guidelines, as well as the Radiation Use Committee, under investigational new drug authorizations from the Food and Drug Administration.

Peripheral blood stem cell mobilization and collection for breast cancer patients. Patients with breast cancer were treated on a protocol using a higher starting dose of 90Y based on prior experience in breast cancer with m170-radioimmunotherapy and autologous peripheral blood stem cell (PBSC) transplantation (29). After fulfilling eligibility, but before imaging or therapy, patients were given granulocyte colony-stimulating factor (10 µg/kg) s.c. daily, and PBSCs were harvested by apheresis starting on day 5 as described previously (30) until a minimum of 2 x 106 CD34+ cells/kg were collected and cryopreserved. Per Food and Drug Administration mandate, the cells were infused only if the patient had prolonged grade 4 hematologic toxicity or neutropenic fever. The on-study date for the breast cancer trial was the day granulocyte colony-stimulating factor was initiated for PBSC collection. PBSCs were not harvested for prostate cancer patients, and the on-study date was the day cyclosporine was initiated.

Imaging and therapy with indium-111/yttrium-90-1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-peptide-m170 with or without paclitaxel. Because 90Y has only ß emissions and is thus not suitable for imaging, 111In was used as a tracer for pharmacokinetic studies, imaging, and calculation of 90Y radiation dosimetry (20, 29, 31–33). See Figure 1 for treatment schematic. After an imaging dose of 111In-DOTA-peptide-m170 (~5 mCi 111In on 5-10 mg m170), serial blood and urine specimens and whole body scans were obtained immediately after injection and at 4, 24, and 48 hours, with one additional study between 72 and 144 hours. Methods for collecting and analyzing the pharmacokinetic and dosimetry data have been described previously (20, 33, 34). One week after the imaging dose, the therapy dose of 90Y-DOTA-peptide-m170 was administered after verification that the imaging/dosimetry showed tumor targeting and no unexpected normal organ uptake. A tracer dose of 111In-DOTA-peptide-m170 was administered concurrently. Prior studies have shown comparable pharmacokinetics and dosimetry of the initial "imaging" and the subsequent administration of 11II in conjunction with 90Y-m170 treatment (20, 29). Imaging and therapy doses were administered in the radioimmunotherapy outpatient facility. Patients were instructed on precautions according to Radiation Use Committee policies. Serial blood counts, chemistries, clinical examinations, and tumor measurements were done following therapy. Criteria for removal from the study included unacceptable toxicity, progressive disease, and HAMA. The starting dose of 90Y was 12 mCi/m2 for prostate cancer and 22 mCi/m2 for breast cancer. Because of our prior experience with high-dose radioimmunotherapy and autologous stem cell transplantation in breast cancer, Food and Drug Administration permitted a higher starting dose for the these patients. The first cohort of patients on each protocol received radioimmunotherapy without paclitaxel. The second cohort received the same dose of radioimmunotherapy as the first cohort, but at 48 hours after radioimmunotherapy the patients in both prostate and breast cancer studies received 75 mg/m2 paclitaxel, a dose level and timing determined from preclinical studies that showed synergistic antitumor effect without additional toxicity in mouse xenograft models of both prostate and breast cancer (35, 36). 90Y dose escalation by 10 mCi/m2 with the same low-dose paclitaxel was planned for the third and subsequent cohorts of patients, unless grade 3 or 4 nonmarrow toxicity was observed in two or more of an extended six-patient cohort, which would define the maximum tolerated dose. Acute toxicity (within 3 days of injection) and delayed toxicity (within weeks to months of injection) were graded according to the National Cancer Institute Common Toxicity Criteria of 1998.



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  		Fig. 1. Treatment schematic: (1) PBSC harvest after granulocyte colony-stimulating factor (G-CSF) mobilization before imaging/therapy for breast cancer patients only; (2) cyclosporine for a total of 28 days, beginning 3 days before imaging, to prevent HAMA; (3) imaging with 111In DOTA-peptide-m170 to determine dosimetry and assure uptake in tumor and no unexpected uptake in normal tissue; (4) 90Y-DOTA-peptide-m170 given 1 week after 111In-imaging at a starting dose of 12 mCi/m2 for prostate and 22 mCi/m2 for breast cancer; (5) second cohort received paclitaxel 48 hours after 90Y-DOTA-peptide-m170 (a time when radiation has concentrated in tumor and cleared normal tissues); (6) PBSC infusion in breast cancer patients if neutropenic fever or >5 days of grade 4 neutropenia, thrombocytopenia.

 
Cyclosporine to prevent human anti-mouse antibody. Previous studies with m170-radioimmunotherapy in prostate cancer showed a 71% frequency of HAMA (37). In an effort to provide information for future trials using multidose fractionated radioimmunotherapy, patients enrolled in the present trial received cyclosporine 5 mg/kg orally every 12 hours beginning 3 days before the imaging dose and continuing for a total of 25 days to reduce HAMA frequency (Fig. 1). Cyclosporine trough levels were measured twice weekly by high-performance liquid chromatography and maintained between 150 and 250 ng/mL. Pretherapy and post-therapy HAMA studies were done using a standard enzyme-linked immunoabsorbent assay as described previously (28, 29).

Radioimmunoconjugates of m170 with a novel tetrapeptide linker, 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic acid-peptide. The antibody used to target prostate and breast tumors was a murine monoclonal IgG1, m170.82, developed using a synthetic asialo-GM1 terminal disaccharide immunogen (related to the Thompson-Friedenreich disaccharide (38–40). M170 binds with high affinity (4 x 108 mol/L–1; ref. 41) to aberrant carbohydrate residues on MUC-1, an epithelial mucin expressed on a variety of adenocarcinoma cells, including prostate and breast cancer. The m170 antibody (Biomira, Inc., Edmonton, Alberta, Canada) was prepared as described previously (30) using good manufacturing practices (current good manufacturing practice grade), was >95% monomeric IgG by PAGE, and met U.S. Food and Drug Administration specifications. Ninety percent of metastases from patients with breast cancer showed abundant immunohistochemical reactivity with m170 (42). In previous trials, scintigraphy showed targeting of prostate and breast cancers (21, 29, 37–39).

DOTA-peptide-m170 (DOTA-glycyl-glycyl-glycyl-Phe-m170) was prepared and purified by methods described previously (33, 43–46). The unlabeled immunoconjugate was labeled with either 111In for imaging or 90Y for treatment. The resulting radioimmunoconjugates were 99% pure by molecular sieving chromatography. Immunoreactivity was comparable with that of the unmodified antibody; cellulose acetate electrophoresis showed that 98% of all radioimmunoconjugates were in monomeric form, and high-performance liquid chromatography indicated that >95% of the radiometal was associated with the radioimmunoconjugates. Radiolabeled DOTA-peptide-m170 was prepared and confirmed to be sterile and pyrogen-free before infusion.

Statistical methods. To test for differences between the DOTA-tetrapeptide linker used in the present study and the 2IT-BAD linker used in prior studies or for differences between prostate and breast cancer patients, a Wilcoxon rank sum test was used. All reported Ps were two tailed.


   Results
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Nine prostate cancer patients had pharmacokinetics and an imaging study (three as part of a Food and Drug Administration–requested imaging-only study before approval of the therapy trial and six received therapy doses of radioimmunotherapy at 12 mCi/m2; Table 1). Three of the six patients were given paclitaxel 48 hours after radioimmunotherapy. Eight breast cancer patients were enrolled. One did not have an adequate stem cell collection and was thus "off study"; the other seven proceeded to pharmacokinetics and imaging. One of these patients developed rapidly progressive disease with spinal cord compression between the day of imaging and the day scheduled for treatment. Six of the seven patients imaged received therapy doses of 22 mCi/m2. Two of the six treated breast cancer patients received paclitaxel 48 hours after radioimmunotherapy.


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  		Table 1. Clinical characteristics of patients with metastatic prostate (patients 1-9) and breast (patients 10-17) cancer imaged/treated with 111In/90Y-DOTA-peptide-m170

 
Pharmacokinetics, imaging, and dosimetry. Blood and urine clearances of 111In-labeled DOTA-peptide were similar in prostate and breast cancer patients and similar to the clearances of 111In-DOTA-2IT-m170 in prostate and breast cancer patients as reported previously (21, 29, 33, 37, 47). Bone or soft tissue metastases from prostate and breast cancer were targeted in 15 of the 16 patients imaged with no unexpected uptake in normal tissues (Fig. 2). Radiation dose (rads/mCi) to the whole body, normal organs, and tumors was calculated from the pretherapy 111In imaging in eight prostate cancer and six breast cancer patients. [One of nine prostate patients imaged had small volume nodal and bone disease that did not image as noted in Table 1 (patient 4). One of the seven breast cancer patients imaged (Table 1, patient 16) became too ill to complete the serial studies needed for dose calculations.] Pharmacokinetics and dosimetry of the initial "imaging" dose of 111In-DOTA-peptide-m170 and the 111In coadministered with 90Y-radioimmunotherapy were comparable as has been observed in our previous studies (20, 29). In Table 2, dosimetry results for patients imaged with the DOTA-tetrapeptide-linked m170 were compared with results obtained using 2IT-BAD-linked m170 in 24 patients studied previously [17 prostate cancer patients (37) and 7 breast cancer patients (29)]. As expected for radiometal-labeled antibodies, the liver was the nonmarrow organ receiving the highest radiation dose. The median liver dose was 10.1 rads/mCi for the nine patients with prostate cancer imaged with the DOTA-peptide linker [dosimetry for eight of these patients reported previously (33)] versus 15.7 rads/mCi in prior studies using the 2IT-BAD linker (37). In the six breast cancer patients described in the present study using DOTA-peptide, the median liver dose was 12.6 rads/mCi compared with 16.7 rads/mCi in a prior study using 2IT-BAD (29). The liver dose was significantly lower using the DOTA-peptide linker for the prostate cancer patients (33) and for the combination of prostate and breast cancer patients (P < 0.001, Wilcoxon rank sum test). When breast cancer patients were evaluated separately, the reduction in liver dose approached statistical significance (P = 0.063). The radiation dose to tumors was not significantly affected by the linker (P = 0.41; refs. 33, 47).



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  		Fig. 2. 111In-DOTA-peptide-m170 targets prostate and breast cancer. A, 111In-DOTA-peptide-m170 targeting at 6 days in a prostate cancer patient. Posterior view of the pelvic region shows areas of known tumor uptake in soft tissue lesions (white arrows) and most of the bones, including left femur, right sacroiliac joint, sacrum, lumbar vertebrae, and multiple sites in the ilia. B, anterior view in a breast cancer patient shows targeting at 6 days of right supraclavicular and infraclavicular nodal metastases (white arrow) and cardiac blood activity (black arrow). In the complete sequence of images in these patients, uptake was seen as early as 1 hour and metastases were well defined by 3 days. 111In activity is shown in color intensity modulated images with hottest colors (white, red, and orange), reflecting greater activity than cooler colors (green and blue).

 

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  		Table 2. Radiation dosimetry comparison of 2IT-BAD and DOTA-peptide-linked-m170

 
Toxicity. Sixteen patients received an imaging dose of 111In-DOTA-peptide-m170 and 12 of those patients received a treatment dose of 90Y/111In-DOTA-peptide-m170. The first cohort of patients received the radioimmunoconjugate alone (three prostate and four breast cancer patients) at doses of 12 mCi/m2 for prostate and 22 mCi/m2 for breast cancer. One of the four breast cancer patients had pulmonary function tests slightly below the entry criteria and was treated but could not be "counted" in the first cohort. The second cohorts received the same dose of radioimmunotherapy followed by paclitaxel at 48 hours after radioimmunotherapy (three prostate and two breast cancer patients). The 28 doses of radioimmunoconjugate were well tolerated. No patient had fever, chills, urticaria, or shortness of breath suggestive of an immunologic reaction to the antibody/radioconjugate. During the first 3 days following the imaging study, two prostate and one breast cancer patients had grade 1 anorexia/nausea that may have been associated with cyclosporine administration. One prostate cancer patient had grade 2 headache possibly secondary to cyclosporine. One breast cancer patient had transient intermittent grade 3 nausea and vomiting before, during, and after her participation in this trial due in part to large doses of narcotic analgesics. Her symptoms did not seem to be related to the study agent(s).

As expected, myelosuppression was the predominant toxicity of the treatment dose of 90Y-DOTA-peptide-m170 (Table 3). Three of the six breast cancer patients treated required reinfusion of stem cells to hasten hematopoietic recovery. One, treated without paclitaxel (Table 1, patient 10), developed bleeding and neutropenic fever in the setting of disease progression and biliary obstruction and two, treated with radioimmunotherapy plus paclitaxel (Table 1, patients 14 and 17), developed neutropenic fever.


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  		Table 3. Hematologic toxicity following 90Y-DOTA-peptide m170

 
Patients treated with paclitaxel had significant but subtotal alopecia. This was noticeable in the prostate cancer patients only, because the breast cancer patients had preexisting alopecia. The only nonmyelosuppressive toxicities above grade 2 included the grade 3 intermittent nausea/vomiting describe above (Table 1, breast cancer patient 11) and that of a 76-year-old prostate cancer patient (Table 1, patient 7) who developed a cardiac arrhythmia requiring hospitalization and eventual ablation of an irritable focus. He had a similar episode of arrhythmia several years before participation in this clinical trial. The stress of grade 1 anemia seems unlikely to be the precipitating event, but the patient was transfused to ameliorate this potential contributing factor. The maximum tolerated dose was not reached, and the study was closed due to slow accrual to an intensive therapy trial in a patient population composed of primarily elderly prostate cancer patients and heavily pretreated breast cancer patients. However, the clinical results indicate that myelosuppression is the limiting toxicity and stem cell support will be necessary to escalate doses to the tolerance of other organ systems using this radioimmunoconjugate in combination with radiosensitizing paclitaxel.

Human anti-mouse antibody. Nine prostate cancer patients who received an imaging dose of m170-DOTA-peptide were followed for 37 to 322 days after exposure and none developed a positive (>5 µg/mL) HAMA. Six breast cancer patients were evaluated for HAMA between 35 and 56 days after exposure to m170-radioimmunotherapy, and only one patient developed a positive HAMA (Table 1, patient 11). The murine protein was the target of the HAMA, not the DOTA-tetrapeptide. This patient who developed HAMA had intermittent vomiting throughout her course (related to narcotic analgesics) and had low cyclosporine levels (<150 ng/mL) for 2 weeks beginning on the day of 90Y-radioimmunotherapy administration. There was no apparent adverse clinical effect of the positive HAMA.

Clinical response. The purpose of these parallel phase I trials was to evaluate pharmacokinetics, imaging, dosimetry, and clinical toxicity of the experimental therapy. However, clinical measures of tumor activity were also monitored (Table 1). Of the six patients with prostate cancer who received a dose of 90Y-DOTA-peptide-m170, two showed a 50% decline in prostate-specific antigen lasting 2 months (Table 1, patients 8 and 9). Two patients described a decrease in bone pain (Table 1, patients 3 and 5). The other patients with prostate cancer had minimal tumor-related pain before therapy. Of six breast cancer patients, all heavily pretreated, who received a therapy dose of 90Y-DOTA-peptide-m170, two patients had minor reduction (<50%) in soft tissue disease lasting <1 month.


   Discussion
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 Results
 Discussion
References
 
Metastatic prostate and breast cancer are radiotherapy-responsive solid tumors. Targeted systemic irradiation is an attractive treatment option that is being routinely implemented in lymphomas, exquisitely radiosensitive tumors. For the treatment of solid tumors, higher radiation doses will be required. Strategies to optimize radiation delivered to tumor while minimizing radiation exposure to normal organs are essential. In these parallel phase I clinical trials in hormone-refractory prostate cancer and metastatic breast cancer, patients were treated with an adenocarcinoma-targeting MUC-1 antibody, m170, attached to the chelated radiometal (111In and/or 90Y) via a novel tetrapeptide linker (Gly3Phe). This peptide linking the antibody, m170, to the macrocyclic chelate of radiometals, DOTA, was shown in vitro to be susceptible to the endopeptidase activity of intrahepatic cathepsins, responsible for most protein metabolism in hepatic lysosomes (45). Catabolism of the peptide linker by hepatic cathepsins facilitates clearance of the radiochelate from the liver in a form quickly excreted by the kidney, thereby reducing the radiation dose to dose-limiting normal nonmarrow tissue, the liver, and enhancing the amount of radiation that can be safely delivered to tumor.

The 111In-radioimmunoconjugate, DOTA-peptide-m170, showed excellent tumor targeting in sites of known bone and/or soft tissue disease in 15 of the 16 patients imaged. From the successful targeting, as seen in Fig. 2 at 6 days after infusion, it is apparent that the cathepsin-degradable peptide linking the radiochelate to the antibody was not degraded by tumor tissue. Only one patient with prostate cancer and small deep nodal disease failed to image adequately. The injections of both 111In-radioimmunoconjugate and 90Y-radioimmunoconjugate were well tolerated with no acute toxicities associated with the infusion. The pharmacokinetics and dosimetry for prostate and breast cancer were similar. Although the dose-limiting toxicity of radioimmunotherapy is marrow suppression, this can be circumvented by PBSC transplantation as shown in this and prior studies (7, 21, 29, 30). When radiometal radioimmunoconjugates are used, the nonmarrow dose-limiting organ is the liver. The efficacy of the DOTA-tetrapeptide linkage in reducing radiation dose to the liver seen here confirms preclinical (45, 46) and initial clinical data in prostate and breast cancer (20, 33, 47). The clinical dosimetry study in prostate cancer (33) showed a 31% reduction in radiation dose to the liver comparing patients treated with DOTA-peptide-radioimmunoconjugate to patients treated with m170 linked to the radioisotope via the standard DOTA-2IT (37), with no significant difference in radiation dose to tumor. Extrapolating estimates for stem cell–supported radioimmunotherapy, based on the dosimetry from the present clinical trial (33, 47) and an estimated liver dose limit of 3,000 cGy from external beam irradiation (48), a dose of 90Y DOTA-peptide m170 delivering 3,000 cGy to the liver would be expected to deliver as much as two to five times this dose to tumor, a dose that would be expected to have clinical efficacy (33).

In spite of the reduction in radiation dose to normal liver with the cathepsin-degradable tetrapeptide linker, meaningful therapeutic results may require additional enhancement. Based on preclinical studies showing synergy between radioimmunotherapy and paclitaxel (35, 36, 49), the second cohort of patients in the present trial received combined modality therapy. The sequence and timing showing maximum synergy in murine models were translated into the clinical trial reported here. Paclitaxel in a dose comparable with that used in the preclinical model was given 48 hours after radioimmunotherapy. Paclitaxel was chosen because it stabilizes microtubule formation resulting in a mitotic block, bcl-2 dysfunction, and activation of apoptosis (35, 50). It is effective in tumors with mutant p53, a common characteristic of advanced prostate and breast cancers (51–53). Paclitaxel has potentiated the antitumor effect of external beam radiation (22–27) and has been used in conjunction with radioimmunotherapy in ovarian carcinoma (54). Further comparison of the tumor dosimetry in patients described in this study has shown that the administration of paclitaxel after radioimmunotherapy actually modestly increased the tumor cumulated radiation, suggesting an additional mechanism for enhancement of antitumor activity with combined modality radioimmunotherapy (55).

Although paclitaxel seemed to increase transient myelosuppression of radioimmunotherapy in the present study, particularly in the heavily pretreated breast cancer patients, there was no apparent other toxicity, except for alopecia. None of the patients exhibited any delayed organ toxicity. Further escalation of the radioimmunotherapy dose in conjunction with fixed-dose paclitaxel is feasible, although it will require PBSC support.

Fractionation is one method for increasing the radiation dose to tumor without excessive toxicity to normal tissue and is the standard approach in external beam radiotherapy. Prior preclinical and clinical studies of radioimmunotherapy have shown reduced toxicity with a fractionated dose schedule (56–58). For example, breast cancer patients tolerated three cycles of 131I-radioimmunotherapy at 1,000 cGy per dose, without the pulmonary or cardiac toxicity observed when single doses >2,700 cGy were used in lymphoma patients receiving 131I-radioimmunotherapy (59) Unfortunately, repeated use of a murine or even partially humanized radioimmunoconjugate may result in formation of antibodies to the targeting antibody that could prevent multidose therapy. In prior studies using m170 in prostate cancer (as 90Y/111In-2IT-BAD-m170), 12 of 17 patients developed HAMA after a single pair of imaging/treatment doses 1 week apart (37). Supporting previous reports (20, 60, 61), only one patient developed HAMA in the present trial using cyclosporine, making fractionated therapy feasible in future trials.

Although bone marrow is the dose-limiting tissue for non–stem cell–supported radioimmunotherapy, the dose to the marrow from high-dose radioimmunotherapy is low compared with the dose of total body irradiation used in conventional myeloablative bone marrow transplantation (~1,200 cGy). Radiopharmaceuticals that specifically target hematopoietic cells have been successfully used in clinical bone marrow transplant trials and can be added to standard total body irradiation and to chemotherapy without apparent increase in nonmarrow toxicity (62, 63). There is no evidence of damage to the marrow stroma or other injury that would interfere with either autologous or allogeneic engraftment at the dose levels used in these trials (64). With PBSC, the radiation dose of 90Y-radioimmunotherapy is limited by the dose to normal liver tissue. Within that limit, combination of radioimmunotherapy with radiation-sensitizing chemotherapy delivered at an optimal time, when it will sensitize the tumor and not normal tissue, has the potential for delivering systemic tumoricidal doses of radioimmunotherapy.

Further developments may improve tumor targeting with reduced toxicity to normal tissues. Molecules smaller than intact immunoglobulins, such as single-chain antibodies, may better penetrate deeper into sites of tumor (65, 66). Pretargeting approaches first target the tumor with antibody and then remove unbound antibody from the bloodstream/tissues before administration of the radioactive small molecule. Radioactivity that does not bind to the tumor-linked antibody is rapidly excreted by the kidney, minimizing nonspecific normal tissue exposure (67). In preliminary trials in non-Hodgkin's lymphoma patients, clinically effective doses were delivered without significant marrow toxicity (68). However, successful treatment of solid tumors will likely require higher radiation doses. Additional approaches, such as radiosensitizing chemotherapy, PBSC support, and fractionated therapy, may be important strategies to enhance the efficacy of the methodologies and targeting agents being developed. The tetrapeptide-linked radiometal-labeled radiopharmaceutical used in this study decreased liver cumulated activity and radiation dose compared with the conventional 2IT-BAD linker, without a change in dosimetry or pharmacokinetics to tumor or other tissues. Additionally, the tetrapeptide linker was not immunogenic in patients treated with cyclosporine to prevent HAMA; thus, multiple dose therapy in this setting is feasible. The addition of paclitaxel, in the dose employed here, did not seem to alter pharmacokinetics, although it did cause alopecia, may have added to hematologic toxicity, and is associated with a modest enhancement of cumulated tumor radiation dose (55). Although the maximum tolerated dose was not reached in these trials, transient evidence suggesting antitumor response was observed. Further studies will be needed to optimize targeted radiation therapy in solid tumors.


   Acknowledgments
 
We thank the clinical staff at University of California-Davis Cancer Center and the Bone Marrow Transplant Unit for their assistance with patient care, G.R. Mirick for performing the HAMA assays, and S. Percey for assistance with article preparation.


   Footnotes
 
Grant support: National Cancer Institute grant PO1 CA47827, National Cancer Institute/NIH grant PO1-CA47829, Veteran's Administration Northern California Healthcare System (R.T. O'Donnell, J.M. Tuscano, and T. Wun), and American Cancer Society Clinical Research Training grant (P.N. Lara).

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Note: Presented in part at the 2004 American Society of Clinical Oncology Annual Meeting, New Orleans, Louisiana.

Received 2/ 7/05; revised 5/11/05; accepted 5/24/05.


   References
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 Discussion
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