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Clinical Cancer Research Vol. 9, 3973S-3981S, September 1, 2003
© 2003 American Association for Cancer Research

Session IV: CLINICAL STUDIES: SOLID TUMORS

Pharmacokinetics and Dosimetry Studies for Optimization of Anti-Carcinoembryonic Antigen x Anti-Hapten Bispecific Antibody-mediated Pretargeting of Iodine-131-labeled Hapten in a Phase I Radioimmunotherapy Trial1

Françoise Kraeber-Bodéré, Alain Faivre-Chauvet, Ludovic Ferrer, Jean-Philippe Vuillez, Pierre-Yves Brard, Caroline Rousseau, Isabelle Resche, Anne Devillers, Sophie Laffont, Manuel Bardiès, Ken Chang, Robert M. Sharkey, David M. Goldenberg, Jean-François Chatal and Jacques Barbet2

Nuclear Medicine Department, René Gauducheau Cancer Center, 44805 Saint Herblain, France [F. K-B., L. F., C. R., I. R., J-F. C.]; INSERM U463, 44093 Nantes, France [F. K-B., A. F-C., M. B., J-F. C., J. B.]; Nuclear Medicine Department, Michalon Hospital, 38043 Grenoble, France [J-P. V., P-Y. B.]; Nuclear Medicine Department, Eugène Marquis Cancer Center, 35062 Rennes, France [A. D., S. L.]; IBC Pharmaceuticals, Inc., Morris Plains, New Jersey 07950 [K. C., D. M. G.]; and Garden State Cancer Center, Belleville, New Jersey 07109 [R. M. S., D. M. G.]


   ABSTRACT
Top
 ABSTRACT
Introduction
 Materials and Methods
 Results
 Discussion
 REFERENCES
 
Purpose: Pharmacokinetics and dosimetry of hMN-14 x m734 bispecific monoclonal antibody (BsMAb) and 131I-labeled di-diethylenetriaminepentaacetic acid-indium (131I-hapten) were studied to optimize pretargeted radioimmunotherapy.

Experimental Design: Thirty-five patients with carcinoembryonic antigen-expressing tumors were included. In a first group of 12 patients, 131I-trace-labeled BsMAb doses were escalated from 10 to 100 mg/m2, and 3.7 GBq of 131I-hapten were administered 7 days later. In a second group, 12 patients received 75 mg/m2 BsMAb and 2.6–4.2 GBq of 131I-hapten 5 days later. The BsMAb dose was then reduced to 40 mg/m2, and 10 patients received 1.9–5.5 GBq of 131I-hapten. Blood samples were collected. Biodistribution was monitored by quantitative scintigraphy.

Results: Directly labeled BsMAb pharmacokinetics was described by two exponentials: half-lives were 8.1 h (2.0–18.1 h) and 48.2 h (22.8–79.4 h); blood clearance was 123 ml/h (64–195 ml/h). With a 7-day interval, 10 or 30 mg/m2 BsMAb resulted in fast elimination and very low tumor uptake of hapten, whereas 50 or 100 mg/m2 resulted in favorable tumor accretion. With 75 mg/m2 BsMAb and a 5-day interval, hapten clearance was 152 ml/h (81–298 ml/h). Calculated radiation dose to tumor was 3.9 Gy/GBq (0.4–22.4 Gy/GBq) for the hapten, compared with 2.0 Gy/GBq (0.3–3.8 Gy/GBq) for the BsMAb, but hematological toxicity prevented dose escalation. Reduction of the BsMAb dose to 40 mg/m2 accelerated hapten clearance to 492 ml/h (113–2544 ml/h) and reduced hematological toxicity without compromising tumor uptake [5.2 Gy/GBq (0.5–12.6 Gy/GBq)].

Conclusions: Optimized BsMAb doses and time interval will allow for the administration of higher, tumoricidal, activity doses.


   Introduction
Top
 ABSTRACT
 Introduction
Materials and Methods
 Results
 Discussion
 REFERENCES
 
Several clinical RIT3 studies have shown high response rates for both myeloablative and nonmyeloablative doses in patients with recurrent or refractory non-Hodgkin’s lymphoma (1) . For solid tumors, the frequency of complete and even partial responses has been low, not reaching 5% in several Phase I/II studies (2) . However, the clinical trials usually enrolled patients with large tumor burden, and with acceptable irradiation of normal tissues, directly labeled monoclonal antibodies did not deliver tumoricidal doses to the radioresistant tumors.

Pretargeting strategies have been proposed to overcome this problem (3) . The AES is a pretargeting technique now being developed by IBC Pharmaceuticals, Inc. (Morris Plains, NJ). It uses a BsMAb and a radiolabeled bivalent hapten (4 , 5) . Increased tumor:normal tissue ratios and reduced toxicity have been demonstrated in animal RIT studies (6) . Two Phase I/II clinical trials assessing a murine anti-CEA x anti-DTPA-indium BsMAb (F6 x 734), composed of two chemically linked Fab' fragments, and iodine-131-labeled di-DTPA-indium hapten, in 38 patients with MTC or SCLC recurrences showed encouraging therapeutic results with six responses and one stabilization (7 , 8) .

To reduce immunogenicity, a chimeric bispecific antibody, composed of a humanized anti-CEA antibody (hMN14) and the murine anti-hapten antibody m734, has been prepared. The blood pharmacokinetics of the hMN-14 x m734 BsMAb and of the bivalent 131I-di-DTPA-indium hapten (131I-hapten) and their biodistribution (monitored by quantitative scintigraphy) are presented here and discussed with respect to the possibility of increasing tumor:normal tissue irradiation ratios and designing optimized doses and administration scheme for AES pretargeted RIT.


   Materials and Methods
Top
 ABSTRACT
 Introduction
 Materials and Methods
Results
 Discussion
 REFERENCES
 
Eligibility Criteria.
Patients who were >18 years of age with histologically proven CEA-positive tumors (colon, rectum, stomach, esophageal, pancreatic, biliary, breast, and lung cancers) and had at least one known tumor site measurable by computed tomography or MRI were eligible. Patients with MTC were eligible only in the second part of the study and were considered in a separate group. All patients had a Karnofsky performance status of >=70% and a minimum life expectancy of 3 months. They were required to be at least 4 weeks beyond any major surgery, external radiotherapy, or chemotherapy. Enrolled patients had normal liver (bilirubin <= 1.5 the upper limit of normal), kidney (creatinine <= 1.5 the upper limit of normal), and hematological (leukocytes >= 4000/mm3, granulocytes >= 2000/mm3, platelets >= 100000/mm3) functions. Plasma CEA was measured by enzyme immunoassay 15 and 7 days before RIT. The protocol was approved by the local ethics committee. All patients gave their signed informed consent.

Antibody Preparation and Testing.
The anti-CEA x anti-DTPA-indium BsMAb hMN-14 x m734 was provided by IBC Pharmaceuticals, Inc. as a 4 mg/ml solution. This antibody was obtained by coupling an equimolar amount of the Fab' fragment of the humanized anti-CEA monoclonal antibody (hMN-14) to the Fab' fragment of the murine anti-DTPA-indium monoclonal antibody (m734) activated by o-phenylene-bismaleimide.

The immunoreactivity of the bispecific conjugate for CEA was evaluated by Superdex 200 size-exclusion high-performance liquid chromatography by measuring the fraction of a radioiodinated sample that is shifted, in the presence of excess CEA, toward shorter retention time as a result of binding to CEA. The immunoreactivity was 85%. The ability of the bispecific conjugate to bind to the radiolabeled peptide was similarly demonstrated on size-exclusion high-performance liquid chromatography by noting the shift of the radiolabeled peptide toward shorter retention time upon adding the bispecific con-jugate.

The bivalent hapten used was N-{alpha}-(diethylenetriamine-N,N,N',N''-tetraacetic acid-N''-acetyl)-tyrosyl-N-{epsilon}-(diethylenetriamine-N,N,N',N''-tetraacetic acid-N''-acetyl)lysine (di-DTPA-TL) obtained by reaction of the dianhydride of DTPA with tyrosyl-lysine diacetate (5) .

Radiolabeling and Quality Control.
The BsMAb was trace-labeled with 370 MBq of iodine-131 in the presence of Iodogen. The radiochemical purity was always better than 95% as checked by instant thin-layer chromatography.

Radiolabeling of hapten with iodine-131 was performed under contract by CIS Bio International (Saclay, France), using the Iodogen method. The specific activity was 38–62 MBq/nmol. The radiochemical purity was >95% by paper chromatography, and immunoreactivity, measured by incubation in m734 antibody-coated tubes, was above 90%.

Antibody and Hapten Infusions.
In study plan 1, 12 patients received i.v. injection with escalating doses (10–100 mg/m2) of 131I-trace-labeled BsMAb hMN-14 x m734 (3 patients/dose level) by infusion over 30–45 min and, 7 days later, with 3.7 GBq of 131I-hapten by infusion over 45 min to 1 h. In study plan 2, 12 patients received 70 mg/m2 of 131I-trace-labeled hMN-14 x m734, and 10 patients received 40 mg/m2 of 131I-trace-labeled hMN-14 x m734 followed by escalating activities of 131I-hapten 5 days later. A PerfuCis pump (CIS Bio International) was used for hapten infusion. Vital signs were monitored before and for 24 h after infusion. Patients were kept in lead-shielded rooms for 5–10 days.

Pharmacokinetics Study.
The blood pharmacokinetics of iodine-131 trace-labeled (370 MBq) hMN-14 x m734 BsMAb was monitored by counting blood samples collected at the end of infusion; at 1 and 4 h after the infusion; and 1, 4, and 5 days after the infusion. The blood pharmacokinetics of the 131I-hapten was determined by counting blood samples collected before infusion and 3, 6, 8, and 10 days after the infusion. The pharmacokinetics of the BsMAb was analyzed according to a two-compartment model, and the pharmacokinetics of hapten was analyzed according to a one-compartment model. Total-body clearance rates were determined by urine collection and whole-body scintigraphy. Urine was collected at three to five separate time points.

Scintigraphy and Dose Rate Measurements.
Whole-body scintigraphy was performed 1 h and 1, 4, and 5 days after trace-labeled BsMAb infusion and 3, 6, 8, and 10 days after hapten infusion. Anterior and posterior views were obtained with a dual-headed camera (DST XL Sophy Camera; Sopha Medical Vision, Buc, France) equipped with a high-energy collimator; the step and shoot method was used. Planar images of the thorax, abdomen, or pelvis were obtained for a few patients using the DST camera equipped with a high-energy collimator. Dose rates were measured at a distance of 1 meter from the patient, using a detection probe, at the end of the hapten infusion, and again at 4 h and repeatedly for 10 days after 131I-hapten administration.

Dosimetry.
Dosimetry was calculated as reported previously (9 , 10) . Briefly, cumulative activity in whole-body, tumor, liver, and kidney was determined from whole-body scintigraphy. Images were corrected for attenuation and dead time and normalized using the image recorded just after administration of the trace-labeled antibody. The positioning of the patient was made reproducible by the use of a system based on laser sources. Radiation doses absorbed by tumor targets and normal tissues were calculated according to the MIRD scheme. The tumor mass was estimated from computed tomography scan sections, and the mass of normal tissues was estimated using reference values.

Toxicity Evaluation.
Toxicity was graded according to Modified National Cancer Institute Common Toxicity Criteria version 2.0. The toxicity in all patients was monitored by clinical examination 15 and 30 days after RIT and at 3, 6, and 12 months; by performing complete peripheral blood cell counts every week during 2 months and at 3, 6 and 12 months posttherapy; and by performing renal and hepatic function evaluations at 15, 30, 45, and 60 days and again at 3, 6, and 12 months posttherapy.

Statistics.
Results are expressed as the arithmetic mean for the relevant patient population or samples with the whole range of observed variation given within parentheses. Pharmacokinetics and dosimetry data were compared among different treatment groups using one-sided Mann-Whitney tests.


   Results
Top
 ABSTRACT
 Introduction
 Materials and Methods
 Results
Discussion
 REFERENCES
 
Patient Characteristics.
Thirty-five patients with CEA-positive tumors were enrolled in this Phase I clinical RIT trial (Table 1)Citation . Twelve patients, 27–71 years of age, with colon, rectum, pancreatic, or lung cancers were enrolled in study plan 1. They received 10, 30, 50, or 100 mg/m2 BsMAb and, 7 days later, 3.7–3.9 GBq of 131I-hapten (except patient 01-05). Twenty-three patients 34–69 years of age with colon, rectum, lung, and pleural cancers (group 2.I; 14 patients) or MTC (group 2.II; 9 patients) were enrolled in study plan 2. Twelve patients received 75 mg/m2 BsMAb followed 5 days later by 2.6–4.2 GBq of 131I-hapten. Because hematological toxicity prevented further hapten dose escalation, the BsMAb dose was reduced to 40 mg/m2, followed 5 days later by 1.9–5.5 GBq of 131I-hapten for 10 patients.


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  		Table 1 Patient characteristics

 
Pharmacokinetics.
After BsMAb administration, most blood pool activity (>90%) was recovered in plasma by centrifugation, and >90% of it was trichloroacetic acid-precipitable at all times. BsMAb blood pharmacokinetics was well described by two exponentials with mean half-lives of 8.1 h (2.0–18.1 h) and 48.2 h (22.8–79.4 h). Mean blood clearance was 123 ml/h (64–195 ml/h; Table 2Citation ). There was no significant correlation between BsMAb blood clearance and BsMAb injected dose (Fig. 1, RCitation 2 = 0.0543) or the concentration of circulating CEA (R2 = 0.0082). Thus, variability is probably the result of the inclusion of patients with different types of cancer (11) and of the inclusion of patients with different disease status.


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  		Table 2 Pharmacokinetics

 


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  		Fig. 1. BsMAb blood pharmacokinetics. Graphs show BsMAb blood pharmacokinetics obtained in study plan 1 (A) with 10 ({square}), 30 ({diamond}), 50 ({triangleup}), and 100 mg/m2 ({circ}) BsMAb and in study plan 2 (B) with 40 ({square}) and 75 mg/m2 ({circ}) BsMAb. Solid lines show best fit with two exponentials of all individual data for each BsMAb dose level.

 
Blood pharmacokinetics of the hapten was more variable (Table 2)Citation . In the 75-mg/m2/5 days group, the mean clearance was 152 ml/h, and the mean terminal half-life was similar to that of the BsMAb. Extending the time interval to 7 days or reducing the BsMAb dose to 40 mg/m2 increased hapten clearance to 491 ml/h (P = 0.05). Hapten clearance is certainly overestimated because calculations are based on late time points only (3 days after hapten administration and later). However, in animals, the distribution phase is fairly short and does not contribute much to the overall area under the curve (12) . Hapten clearance was found inversely related to the BsMAb blood concentration at the time of hapten administration (R2 = 0.318).

With a 7-day interval, low BsMAb doses (10 and 30 mg/m2) resulted in fast elimination and very low tumor uptake for the hapten (Fig. 2A)Citation . With doses of 50 and 100 mg/m2 and a 7-day interval, measurable tumor accretion of the hapten was observed (Fig. 2B)Citation . As expected, hapten tumor uptake appeared to correlate with BsMAb uptake in the tumor at the time of hapten infusion, but the calculation was not possible for 12 of the 21 patients because tumor uptake or tumor mass could not be estimated.



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  		Fig. 2. Hapten biodistribution in study plan 1. BsMAb doses (10 and 30 mg/m2) and 7 days of pretargeting time resulted in very low tumor uptake for the hapten. With doses of 50 and 100 mg/m2 and a 7-day interval, favorable tumor accretion of the hapten was observed. The figure (A: left, anterior view; right, posterior view) shows an example (patient 01-01) of whole-body scan with a 10-mg/m2 dose (blood concentration at the time of hapten infusion, 1.8 nM), 2 days after hapten infusion: no hapten tumor targeting was observed. The remaining activity was low and present mainly in the excretion organs (kidneys and intestine). Activity seen in the intestine moved along the gastrointestinal tract with time. B (left, anterior view; right, posterior view) shows images recorded 5 days after hapten infusion in another patient (01-09) treated with 50 mg/m2 BsMAb (blood concentration at the time of hapten infusion, 7.0 nM). The patient had multiple metastases in lungs, mediastinum, and liver. The metastases showed high hapten uptake. Arrows show some tumor sites.

 
Hapten uptake in tumor was also well correlated with BsMAb blood concentration at the time of hapten administration. In study plan 1, detectable hapten targeting was obtained when this concentration was between 4.4 and 19.8 nM (mean, 10.7 nM). No hapten targeting was observed when it was between 0.7 and 2.8 nM (mean, 1.4 nM). The concentration of BsMAb in blood at the time of hapten infusion was thus considered as a useful parameter for the optimization process.

With 75 mg/m2 BsMAb and a 5-day interval, measurable tumor uptake of the hapten was observed. The slow blood clearance of hapten indicated that the BsMAb dose was too large, or the delay was too short (Table 2)Citation . These observations are consistent with the blood concentration of BsMAb at the time of hapten infusion: 23.4 nM (19.4–38.1 nM). Reduction of the BsMAb dose to 40 mg/m2 accelerated hapten clearance without compromising tumor uptake (Table 2)Citation . Again, this is consistent with the blood concentration of BsMAb measured at the time of hapten infusion under these conditions [18.1 nM (14.4–21.6 nM)]. Fig. 3Citation shows an example of scintigraphy obtained with 40 mg/m2 BsMAb and the 5-day pretargeting interval.



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  		Fig. 3. Optimized pretargeting. Imaging was performed in a patient with neck and chest lymph nodes and bone metastases of MTC. This patient (03-34) received with 40 mg/m2 BsMAb and 2.6 GBq of 131I-hapten injected 5 days later. Tumor effective period (T1/2E) of BsMAb was 3.2 days. Tumor T1/2E of hapten was 6.1 days. A, anterior view recorded 5 days after injection of the radiolabeled BsMAb. B, anterior view recorded 6 days after 131I-hapten administration. C, MRI performed before RIT. Arrows show a bone metastasis of 1.8 g.

 
The BsMAb terminal effective half-lives (T1/2E) in tumor whole-body, liver, and kidneys were consistent and did not change significantly with BsMAb dose (Table 2)Citation . T1/2E was slightly shorter in the liver and longer in the tumor sites.

Hapten terminal effective half-lives (Table 2)Citation were longer than those of BsMAb in whole-body measurements and in the tumor, where the difference was significant (P < 0.05). This reflects the ability of the bivalent hapten to cross-link two BsMAb molecules. In the tumor, cooperative binding may further increase the stability of hapten tumor accretion according to the principle of AES. Hapten terminal effective half-lives were also longer in the liver and increased with the blood concentration of BsMAb at the time of hapten injection. They were comparably shorter in kidneys.

Dose rate at 1 m from patients was monitored while patients remained in lead-shielded rooms. The area under the measured dose rate at 1 m versus time curve was calculated. A good correlation (R2 = 0.875) was observed between the blood concentration of BsMAb at the time of hapten infusion and the ratio of the cumulated dose to the injected activity (Fig. 4)Citation . This is another way to demonstrate that circulating BsMAb slows the clearance of the hapten down.



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  		Fig. 4. Correlation between cumulated radiation emission and BsMAb concentration in blood at the time of hapten infusion. The dose rate at 1 m from the patient was repeatedly measured after hapten infusion using a {gamma} probe. The curves were fitted with one or two exponentials, and the areas under the curve (cumulated dose at 1 m expressed in mSv) normalized to the hapten injected dose were plotted against the BsMAb concentration measured in blood at the time of hapten infusion ({blacksquare}). The straight line is the result of the linear regression. The correlation coefficient (R2) is 0.875 after exclusion of one patient ({square}).

 
Dosimetry.
Dosimetry calculations for both the BsMAb and the hapten are presented in Table 3Citation . There was no measurable tumor uptake for very low blood concentration of BsMAb at the time of hapten infusion (below approximately 4 nM); above about 5 nM, tumor uptake was measurable in 20 of 26 assessable patients (77%), varying from a low relative dose of 0.4 Gy/GBq to high values up to 22.4 Gy/GBq. With 75 or 40 mg/m2 BsMAb and a 5-day interval, radiation doses delivered to tumor sites were higher for the hapten than for the BsMAb, whereas those delivered to normal organs were comparable (Table 3)Citation . This was statistically significant for both dose groups taken together (P < 0.05) and for the 40 mg/m2/5 days group (P = 0.05). In addition, in both cases, the number of measurable tumor sites was higher with the hapten (n = 29) than with the BsMAb (n = 18). With 40 mg/m2 BsMAb, the doses delivered by the hapten to the tumor were higher, and those delivered to normal organs were lower the than the doses delivered with 75 mg/m2 BsMAb. However, because of the large interindividual variability, the differences were not statistically significant. Also, tumor dosimetry was not significantly correlated to the blood concentration of BsMAb at the time of hapten infusion (Fig. 5)Citation .


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  		Table 3 Dosimetry

 


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  		Fig. 5. Tumor dosimetry as a function of BsMAb concentration in blood at the time of hapten infusion. Tumor radiation normalized to the hapten injected activity was plotted against the BsMAb concentration in blood at the time of hapten infusion: {square}, 40 mg/m2/5 days; {blacksquare}, 75 mg/m2/5 days; {circ}, escalating BsMAb dose/7 days.

 
Tumor:normal tissue dose ratios may be used to appreciate targeting specificity (Table 3)Citation . The BsMAb dose did not significantly change the BsMAb dose ratios, which were high for whole body, moderate in the liver, and poor in the kidneys. This is consistent with the 100-kDa molecular mass of the BsMAb. Tumor:whole-body dose ratios were higher for the hapten than for the BsMAb. This was statistically significant for both dose groups taken together (P < 0.05) and for the 40 mg/m2/5 days group (P = 0.05). The hapten tumor:whole-body dose ratio was significantly higher in the 40 mg/m2 group/5 days (P = 0.05). Despite the long T1/2E observed in the liver, tumor:liver dose ratios were as good as (or even better than) that of the BsMAb. Tumor:kidneys dose ratios were higher than those observed with the BsMAb (P < 0.05).

Hematological Toxicity.
Table 4Citation reports the leukocytopenia or thrombocytopenia events observed after 30 assessable treatments. In study plan 1, hematological toxicity was very low with only one grade III for one of the three patients treated with 100 mg/m2/7 days. At 75 mg/m2 BsMAb and a 5-day interval (study plan 2), hematological toxicity was higher, with several cases of grade III-IV toxicity that prevented dose escalation. With 40 mg/m2 BsMAb and a 5-day interval, hematological toxicity was reduced. Toxicity was higher in group II (MTC) than in group I (non-MTC).


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  		Table 4 Hematological toxicity

 
A high homogeneous or heterogeneous diffuse uptake in the pelvic and spinal skeleton was observed for several MTC patients (Fig. 6)Citation . In addition, most MTC patients (eight of nine patients) had either bone metastases or evidence of bone marrow involvement shown by MRI (Fig. 6)Citation .



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  		Fig. 6. Bone marrow uptake observed in a MTC patient. A, MRI of patient 01-20 showing multiple bone marrow metastases. B, scintigraphy (posterior view) showing bone marrow uptake after injection of 75 mg/m2 BsMAb and 3.0 GBq of 131I-hapten 5 days later.

 

   Discussion
Top
 ABSTRACT
 Introduction
 Materials and Methods
 Results
 Discussion
REFERENCES
 
Pretargeting techniques are considered as promising for RIT of solid tumors, but they require careful optimization (12) . As opposed to pretargeting approaches using the biotin-avidin system, AES has been used thus far as a two-step technique, which means that no chase agent was used. This may be advantageous, because the clinical development of a chase agent is an additional burden: it may prove immunogenic; and its dose and administration schedule must also be optimized (13) . Conversely, the administration parameters of AES must be carefully fine-tuned to achieve a good compromise between tumor accretion and clearance of the labeled hapten. In early animal experiments and immunoscintigraphy clinical trials, the hapten was used at low molar doses, and it proved easy to obtain good tumor:nontumor contrast ratios, with the bivalent nature of the hapten allowing, at the same time, protracted binding in the tumor and fast clearance from normal tissues. For RIT applications, molar doses must be increased, and the determination of the most appropriate doses and time intervals was essential to deliver tumor-ablative activity doses in animals (14) . Then, a first series of clinical trials was based on an extrapolation from mice to man that worked reasonably well. With this new product, pharmacokinetic and dosimetry studies were planned to collect the necessary information for optimization of AES RIT.

Study plan 1 showed that an important parameter was the concentration of BsMAb in the circulation at the time of hapten injection. Below a threshold of about 4 nM, most hapten was cleared rapidly, and little if any tumor uptake observed. The second important information was that whole-body exposure per GBq administered, measured from whole-body images or from dose rate measurements, increased almost linearly with this concentration. Blood data also showed a similar relationship, with hapten clearance decreasing with the concentration of BsMAb in blood at the time of hapten infusion. Relative kidney uptake was essentially unchanged, whereas relative liver uptake increased slowly with the blood concentration of BsMAb at the time of hapten injection. Thus, the maximum tolerated activity of the hapten is expected to decrease in direct relationship with this concentration. Study plan 2 confirmed this conclusion: hematological toxicity observed with 75 mg/m2 BsMAb/5 days, which corresponded to blood concentrations between 19.4 and 38.1 nM, was more severe, for similar injected hapten activities, than that seen with the 40 mg/m2/5 days, which corresponded to blood concentrations between 14.4 and 21.6 nM. The pretargeting time interval directly controlled the concentration of BsMAb in the blood at the time of hapten infusion; similar blood concentrations were obtained 7 days after an infusion of 100 mg/m2 BsMAb or 5 days after an infusion of 40 mg/m2 BsMAb, and whole-body and normal organ exposure was similar (Table 3)Citation .

Finally, the most important parameter is the radiation dose delivered to the tumor. This was also analyzed with respect to the blood concentration of BsMAb at the time of hapten infusion: below a threshold around 4–10 nM, relative tumor uptake was low. Above about 10 nM, hapten relative tumor uptake was similar or higher than that of the trace-labeled BsMAb, but there was no clear relationship between hapten relative tumor uptake and the concentration of BsMAb in the blood at the time of hapten infusion (Fig. 5)Citation . This is in part the result of very variable tumor uptake from one patient to another and from one tumor site to another. However, it is expected that hapten tumor accretion would not increase indefinitely with the BsMAb blood concentration. Thus, it seems logical to target a 10–20-nM range for this concentration to make a good compromise between tumor and normal organ irradiation. In accordance with dosimetry calculations, the maximum tolerated dose of hapten has not been reached in the 40 mg/m2/5 days group, at least for those patients who show no bone marrow uptake.

In this trial, as well as in earlier clinical trials with the AES, high diffuse uptake in the pelvic and spinal skeleton was observed in several MTC patients (7) and was much less frequent in patients with other CEA-expressing tumors. This was not associated with circulating CEA levels or the binding specificity of the anti-CEA antibody because the same observation was made using hMN14 in this study and F6, the murine anti-CEA used previously. Most MTC patients in these studies had either bone metastases or evidence of bone marrow involvement by MRI. Thus, the hematotoxicity could be explained by early and frequent metastatic involvement of the bone marrow by MTC, possibly preceding that of cortical bone that is more easily visualized by conventional imaging techniques. In further clinical trials, because MTC appears to be a highly osteophilic cancer, such patients will be considered separately from other tumors, and the specificity of bone marrow accretion will be documented further.

In conclusion, pharmacokinetic and dosimetry studies allowed us to rationalize the influence of BsMAb doses and of the pretargeting interval on tumor uptake and normal organ exposure. In this trial, antitumor efficacy was limited, but follow-up has not been completed yet for all included patients. This will be the topic of a separate report, including an analysis of immunogenicity. The best compromise between toxicity and tumor uptake has been obtained thus far with 40 mg/m2 BsMAb and a 5-day interval, but a higher BsMAb dose associated with a longer pretargeting time (e.g., 75 mg/m2 and a 7-day pretargeting interval) is expected to give similar results. Under these conditions, the treatment with doses up to 5.5 GBq was well tolerated in the absence of bone marrow involvement. Optimized conditions should allow radiation doses in the 30–70 Gy range, which may be potentially tumoricidal, to be safely delivered. Thus, hapten dose escalation should be possible and should achieve higher antitumor efficacy.


   FOOTNOTES
 
1 Presented at the "Ninth Conference on Cancer Therapy with Antibodies and Immunoconjugates," October 24–26, 2002, Princeton, NJ. Back

2 To whom requests for reprints should be addressed, at INSERM U463, Institut de Biologie, 9 quai Moncousu, 44093 Nantes Cedex 1, France. Phone: 33-2-40-08-47-47; Fax: 33-2-40-35-66-97. Back

3 The abbreviations used are: RIT, radioimmunotherapy; BsMAb, bispecific antibody; CEA, carcinoembryonic antigen; AES, Affinity Enhancement System; MTC, medullary thyroid carcinoma; MRI, magnetic resonance imaging; DTPA, diethylenetriaminepentaacetic acid; SCLC, small cell lung carcinoma. Back


   REFERENCES
Top
 ABSTRACT
 Introduction
 Materials and Methods
 Results
 Discussion
 REFERENCES
 

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