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Pharmacokinetics, Dosimetry, and Toxicity of the Targetable Atomic Generator, ²²⁵Ac-HuM195, in Nonhuman Primates

¹ Department of Molecular Pharmacology and Chemistry, Memorial Sloan-Kettering Cancer Center, New York, New York
² Department of Radiology, John Hopkins University, Baltimore, Maryland
³ Department of Pediatrics, Memorial Sloan-Kettering Cancer Center, New York, New York

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Short-lived -emitting isotopes individually conjugated to monoclonal antibodies have now reached human use, but little is still known about their toxicity. Use of antibody targetable ²²⁵Ac nanogenerators is a new approach in the field of -immunotherapy offering the advantage of a 10-d half-life (t_1/2) and increased potency due to generation of 3 new atoms, yielding a total of 4 -particles. However, the 3 -emitting daughter elements generated have the potential for significant toxicity as these nuclides are no longer bound to the carrier IgG. Methods: Cynomolgus monkeys were used to evaluate the toxicity of prototype ²²⁵Ac nanogenerators. Monoclonal antibody HuM195 (anti-CD33) is the carrier for planned human clinical trials of ²²⁵Ac; there are no CD33 sites in cynomolgus monkeys. In one experiment, 2 monkeys received a single intravenous dose of ²²⁵Ac-HuM195 at 28 kBq/kg. This dose level is approximately the planned initial human dose. In another experiment, 2 animals received a dose escalation schedule of 3 increasing ²²⁵Ac-HuM195 doses with a cumulative activity of 377 kBq/kg. The whole-blood t_1/2 of ²²⁵Ac, ratios of ²²⁵Ac to its ultimate -emitting daughter nuclide ²¹³Bi, generation of monkey anti-HuM195 antibodies (MAHA), hematologic indices, serum biochemistries, and clinical parameters were measured. Monkeys were euthanized and examined histopathologically when the dose escalation reached toxicity. Results: The blood t_1/2 of ²²⁵Ac-HuM195 was 12 d, and 45% of generated ²¹³Bi daughters were cleared from the blood. MAHA production was not detected. Approximately 28 kBq/kg of ²²⁵Ac caused no toxicity at 6 mo, whereas a cumulative dose of 377 kBq/kg caused severe toxicity. In the cumulative dosing schedule, single doses of 37 kBq/kg resulted in no toxicity at 6 wk. After 130 kBq/kg were administered, no toxicity was observed for 13 wk. However, 28 wk after this second dose administration, mild anemia and increases of blood urea nitrogen and creatinine were detected. After administration of an additional 185 kBq/kg, toxicity became clinically apparent. Monkeys were euthanized 13 and 19 wk after the third dose administration (cumulative dose was 377 kBq/kg). Histopathologic evaluation revealed mainly renal tubular damage associated with interstitial fibrosis. Conclusion:²²⁵Ac nanogenerators may result in renal toxicity and anemia at high doses. The longer blood t_1/2 and the lack of target cell antigens in cynomolgus monkeys may increase toxicity compared with human application. Therefore, a dose level of at least 28 kBq/kg may be a safe starting dose in humans. Hematologic and renal function will require close surveillance during clinical trials.

Key Words: ²²⁵Ac • radioimmunotherapy • -particles • CD33 • HuM195

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Radioimmunotherapy is a promising approach to selectively treat a variety of cancers (1,2). It takes advantage of a growing number of monoclonal antibodies to target tumor cells preferentially while sparing normal, healthy tissue. This potentially increases the therapeutic index compared with other treatment modalities. Recent advances in chemistry have led to methods for stable conjugation of a variety of radionuclides, including both halogens and metals to proteins and peptides (3–5).

Radiation as a cytotoxic agent has several advantages over therapeutics that interfere chemically with their target cells. Most importantly, radiation is able to kill tumor tissue without the need for binding or internalization into every target cell. Second, combinations of radiotherapy with some chemotherapeutic approaches results in a more than additive therapeutic effect (6). Due to optimizations in external beam therapy, local control of many tumors is possible, whereas micrometastatic disease is currently amenable only to treatment with systemic chemotherapeutic agents. One important step toward feasible radiotherapeutic treatment of micrometastatic disease is to target radiation via monoclonal antibodies directly to tumor throughout the body. Commonly used isotopes for this are the ß-emitting isotopes ¹³¹I and ⁹⁰Y (7). Recently, several -emitting isotopes such as ²¹³Bi and ²¹¹At have been studied clinically as radioisotopes in cancer therapy (3,8). -Particles are characterized by 400-fold greater linear energy transfer than ß-particles. Whereas the ß-particle’s range is typically between 0.5 and 8 mm, -particles have a tissue pathlength of only 40–90 µm. Therefore, -radiation is suitable for killing single-cell diseases and small tumor cell clusters consisting only of a few thousand cells. One of the most potent, yet challenging, therapeutic -emitting approaches is the use of an -generator: ²²⁵Ac. ²²⁵Ac has the convenient half-life (t_1/2) of 10 d and decays via a chain, yielding a total of 4 -particles. The -emitting daughter nuclides are short-lived (²²¹Fr, t_1/2 = 4.8 min; ²¹⁷At, t_1/2 = 32.3 ms; ²¹³Bi, t_1/2 = 45.6 min), leading to an extremely high cytotoxicity when retained in or near tumor cells or tumor cell clusters. A key drawback of this same property is the release of the daughters from ²²⁵Ac immunoconjugates; such daughters may not be retained in tumor tissue (9). Though used successfully in small animal systems, ²²⁵Ac generators have not been used in humans (10).

In this study, we describe initial monkey studies of a new radioimmunoconjugate consisting of the -emitting isotope ²²⁵Ac conjugated to the anti-CD33 antibody HuM195. HuM195 is a humanized antibody that targets CD33 on acute myeloid leukemia cells. HuM195 has been used as the targeting moiety in a variety of clinical trials as a native antibody and conjugated to ß-emitting and -emitting isotopes (8).

The longest lived -emitting daughter nuclide of ²²⁵Ac is ²¹³Bi. Bismuth accumulates in renal tissue and, therefore, nephrotoxicity was hypothesized to be the dose-limiting toxicity in ²²⁵Ac immunotherapy due to constant kidney accumulation of ²¹³Bi arising from systemic ²²⁵Ac (11). Therefore, prior to human trials, we evaluated ²²⁵Ac-HuM195 toxicity in a nonhuman primate model.

	MATERIALS AND METHODS

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Preparation of ²²⁵Ac-HuM195
²²⁵Ac was conjugated to HuM195 antibody using a 2-step labeling method as described previously (12). Briefly, labeling and preparation of the doses were done at the Memorial Sloan-Kettering Cancer Center (MSKCC) Central Isotope Laboratory to ensure consistency and to standardize each step for human clinical use. ²²⁵Ac (37 MBq in 25 µL 0.2 mol/L HCl; Oak Ridge National Laboratory) was incubated with l-ascorbic acid (150 g/L, 20 µL), 2-(p-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA-NCS) (10 g/L, 50 µL; Macrocyclics), and tetramethylammonium acetate (2 mol/L, 50 µL; Aldrich Chemical Co.) to facilitate incorporation of ²²⁵Ac into DOTA. The reaction was allowed to continue for 30 min at 60°C (pH 5.0). For conjugation of ²²⁵Ac-DOTA to HuM195 (the second-step reaction), another 20 µL of ascorbic acid were added before adding 1 mg of HuM195 (200 µL; Protein Design Laboratories). The pH was adjusted with carbonate/bicarbonate buffer (1 mol/L, 100 µL) to 9.0 and incubation was for 30 min at 37°C. Afterward, free ²²⁵Ac along with other metals was absorbed with 20 µL 10 mmol/L diethylenetriaminepentaacetic acid (DTPA) and the unconjugated ²²⁵Ac was separated from ²²⁵Ac-HuM195 by PD10 size exclusion (Bio-Rad) using 1% human serum albumin in 0.9% saline as eluent. Quality control of the final product included thin-layer chromatography to determine radiopurity, a cell-based binding assay to measure immunoreactivity of the antibody vehicle, Limulus amebocyte lysate testing to determine pyrogen content, and microbiologic culture in fluid thioglycollate of soybean-casein digest medium to verify sterility.

Nonhuman Primate Study Design
Four male cynomolgus monkeys, 2–4 y old, were used in this study. Housing and care were in accordance with the Animal Welfare Act and the Guide for the Care and Use of Laboratory Animals (13). Animal protocols were approved by the Institutional Animal Care and Use Committee at MSKCC. All procedures were performed under short-term intramuscular ketamine anesthesia.

In one experiment, 2 animals received a single intravenous dose of ²²⁵Ac-HuM195 at 28 kBq/kg via femoral venipuncture. This dose level was scaled from 1/20 of the maximum tolerated dose values determined in rodents (10). With regard to the total number of -particles emitted until total decay of the ²²⁵Ac, this dose was similar to a ²¹³Bi dose that has proven safe in patients previously (8).

Animals were observed twice daily for levels of brightness, alertness, and reactivity and weighed weekly. Biochemical and hematologic indices of the blood (IDEXX Laboratory) as well as blood radioactivity values were determined on days 1, 5, 8, 14, and 18 and monthly thereafter.

In another experiment, 2 animals received a dose escalation schedule of 3 increasing ²²⁵Ac-HuM195 doses (at 0, 6, and 34 wk) with a cumulative activity of 370 kBq/kg, also administered by femoral venipuncture. Animals were followed similarly for laboratory values and for clinical toxicity.

Determination of Monkey Antihuman Antibodies
Monkey antihuman antibody production was assessed on days 6 and 12 after the first injection and on day 6 after the third injection in monkeys 3 and 4. Serum samples (100 µL) were incubated with 250 ng/mL ¹¹¹In-HuM195 for 10 min at room temperature and then the molecular weight of the radioactive fraction was determined using Beckman Coulter System Gold high-performance liquid chromatography (HPLC) equipped with a -RAM Radio HPLC detector (IN/US Systems, Inc.). Therefore, 50-µL samples were injected onto a size-exclusion column (300 x 7.8 mm TSK 3000SWXL; Supelco) and an elution profile was measured over 16 min. The mobile phase consisted of 0.15 mol/L sodium chloride/0.02 mol/L sodium acetate, pH 6.5. The sensitivity of the HPLC radiodetector and the ¹¹¹In-HuM195 antibody concentration allows accurate detection of monkey anti-HuM195 antibody (MAHA) titer above 50 ng/mL.

Histopathologic Evaluation
Necropsy was performed after euthanasia by barbiturate overdose. Tissues from the cerebrum, testis, salivary gland, skeletal muscle, esophagus, trachea, stomach, small intestine, spleen, lung, liver, adrenal grand, large intestine, and kidneys were stained with hematoxylin–eosin and examined by 2 veterinary pathologists.

Pharmacokinetic Analysis and Absorbed-Dose Estimation
Due to the concern of iatrogenic anemia, blood samples were kept to a minimum and only blood samples of 1 mL were analyzed for blood activity level by -counting (Packard Cobra -Counter; Packard Instrument Co.). The blood pharmacokinetic data obtained in the ²²¹Fr and ²¹³Bi photon windows were used to estimate blood and renal cortex absorbed dose. Starting at 30 min after each blood drawing, blood activity was measured separately for ²¹³Bi and ²²¹Fr and measured several times thereafter until equilibration of the daughters. The steady-state ratio of ²¹³Bi to ²²¹Fr was back-extrapolated by using exponential growth of ²¹³Bi and constant activity of ²²¹Fr. Due to the 4.5-min t_1/2 of ²²¹Fr, its activity approximates ²²⁵Ac after 25 min. In this way, time–activity curves for ²²⁵Ac and ²¹³Bi were obtained separately.

The blood dose was obtained by fitting the ²²⁵Ac time–activity curve to a 1-component (monkey 2) or 2-component (monkey 1) exponential expression. The resulting fitted expressions were analytically integrated over time to yield the total number of ²²⁵Ac disintegrations occurring in the blood. Fitting was performed using the Simulation Analysis and Modeling software package, SAAM II (SAAM Institute, Inc.). Two different assumptions were then applied to estimate the number of ²²¹Fr and ²¹⁷At disintegrations in blood. In the first assumption, these 2 daughters were assumed to decay at the site of ²²⁵Ac decay—that is, in the blood. In the second assumption, these daughters were assumed to decay at the site of ²¹³Bi decay. ²¹³Bi decays not occurring in the blood were assumed to occur in the kidneys, specifically the renal cortex portion of the kidneys. Once the decays of each radionuclide were apportioned to blood or the renal cortex of the kidneys, the absorbed dose was obtained assuming a uniform deposition of -particle energy in the tissue mass. The energy emitted per disintegration was, therefore, multiplied by the number of disintegrations and by the -particle energy emitted per disintegration; the result was then divided by the blood or renal cortex mass. The electron and photon energy contributions to the tissue absorbed dose were considered negligible relative to the -particle energy and were, therefore, not included.

The number of ²¹³Bi disintegrations occurring in the blood was obtained separately by integrating, over time, the back-extrapolated estimate of ²¹³Bi radioactivity concentration in blood at each time. This provided an estimate of ²¹³Bi disintegrations occurring in the blood. The difference between the total number of ²²⁵Ac disintegrations and the ²¹³Bi disintegrations—that is, those disintegrations resulting from ²¹³Bi that was no longer in the blood—was assigned to the renal cortex portion of the kidneys (14). The decay of an equal number of ²²¹Fr and ²¹⁷At was assigned to the blood (assumption A) or to the renal cortex of the kidneys (assumption B).

The mass of blood in cynomolgus monkeys was obtained as the product of 80 mL/kg and the total body weight of the monkeys (15). The mass of the renal cortex was obtained by assuming that the kidney mass of a 5-kg cynomolgus monkey is the same as that of a 5-kg (2–3 y old) human (16). The adult renal cortex-to-total kidney mass ratio (17) was then used to obtain the renal cortex mass.

	RESULTS

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Toxicity of Single-Dose ²²⁵Ac-HuM195
At a dose level of 28 kBq/kg administered once intravenously, no clinical or laboratory toxicity was observed for >6 mo. Serum tests including hepatic enzymes (serum glutamic-oxalocetic transaminase, serum glutamic-pyruvic transaminase, alkaline phosphatase, -glutamyl transpeptidase, and bilirubin), renal values (blood urea nitrogen [BUN] and creatinine), hematology values (hemoglobin, red blood cell count, and white blood cell [WBC] count), and electrolytes (sodium, potassium, chloride, carbon dioxide, calcium, and phosphorus) were normal at all times. A nonspecific rise in serum creatine phosphokinase (CPK) was observed after intramuscular anesthesia. Independence of this finding from radioimmunoconjugate administration was shown by elevated CPK values after each handling, even if no drug was administered. The peak of CPK elevation was reproducibly observed 24 h after intramuscular ketamine anesthesia with significant elevation over 4–5 d.

Blood t_1/2 of ²²⁵Ac-HuM195
The blood t_1/2 of ²²⁵Ac-HuM195 in both monkeys was 12 d. In monkey 1, biphasic pharmacokinetics were observed comprising a rapid clearing component with a t_1/2 of 2.3 ± 0.2 d and a slowly clearing component with a t_1/2 of 12 ± 1 d. In monkey 2, a single-component exponential fit demonstrated a t_1/2 of 12 ± 2 d (Fig. 1).

View larger version (8K): [in this window] [in a new window]   FIGURE 1. Blood time–activity curves of 225Ac. Activity concentration for 225Ac was estimated from 221Fr counting after equilibration. Dashed line (monkey 1, ) corresponds to 2-component exponential fit to data. -Component (42%) cleared with t1/2 of 2.3 ± 0.2 d and slow ß-component (58%) cleared with t1/2 of 12 ± 1 d. Solid line (monkey 2, ) is single-component exponential fit with intercept and t1/2 of 60.3 percentage injected dose per kilogram (%ID/kg) and 12 ± 2 d, respectively.

View larger version (35K): [in this window] [in a new window]   FIGURE 2. HPLC assay for monkey antihuman antibody response (MAHA). 111In-HuM195 was incubated with monkey serum obtained 6 d after injection 1 (top row), 12 d after injection 1 (middle row), and 6 d after injection 3 (bottom row). Antibody dimers (300 kDa) or multimers (>450 kDa) suggestive of autoantibodies against HuM195 were not detected. Retention time of 150-kDa HuM195 is 8.4 min. Higher molecular weight 300 kDa would have markedly shorter retention time (<6.4 min). (Left) Data for monkey 4. (Right) Data for monkey 3.

View larger version (8K): [in this window] [in a new window]   FIGURE 3. Hemoglobin levels in monkeys receiving dose escalation of 225Ac-HuM195. Doses were given at weeks 0, 6, and 28. Arrows indicate time of intravenous injection of 0.15, 0.67, and 1.07 MBq in monkey 3 () and 0.3, 0.67, and 0.93 MBq in monkey 4 ().

View larger version (9K): [in this window] [in a new window]   FIGURE 4. Serum creatinine levels in monkeys receiving dose escalation of 225Ac-HuM195. Dose and schedule were as in Figure 3. Arrows indicate intravenous injection times. , Monkey 3; , monkey 4.

View larger version (9K): [in this window] [in a new window]   FIGURE 5. Serum BUN levels in monkeys receiving dose escalation of 225Ac-HuM195. Dose and schedule were as in Figure 3. Arrows indicate intravenous injection times. , Monkey 3; , monkey 4.

View larger version (8K): [in this window] [in a new window]   FIGURE 6. WBC counts in monkeys receiving dose escalation of 225Ac-HuM195. Dose and schedule were as in Figure 3. Arrows indicate intravenous injection times. , Monkey 3; , monkey 4.

View larger version (82K): [in this window] [in a new window]   FIGURE 7. Histopathologic examination of kidneys taken after euthanasia of monkeys. Kidney damage was observed in different stages. (Left) Monkey 3 shows tubular necrosis and regeneration mixed with some interstitial fibrosis, indicative of more chronic damage. (Right) Monkey 4 also displays tubular damage and marked interstitial fibrosis in more acute state.

Pharmacokinetic Analysis and Absorbed Dose Estimation
Dose estimates were made based on the time–activity curves for ²²⁵Ac (Fig. 1) and ²¹³Bi (Fig. 8). Steady-state in vivo ratios of ²¹³Bi to its parent nuclide ²²⁵Ac was 0.57:1 (±0.11) in monkey 1 and 0.54:1 in monkey 2 (±0.13) (Figs. 8A and 8B). Tables 1 and 2 summarize the dosimetry results obtained under the 2 different assumptions described in the Materials and Methods. Blood and renal cortex absorbed doses were obtained by an extrapolation of kinetics measured for monkey 1 to monkey 3 and monkey 4, both of whom received a total of 1.89 MBq ²²⁵Ac-HuM195. Under the assumption that ²²¹Fr and ²¹⁷At decay with the ²²⁵Ac, the blood dose is 4 Gy and the renal cortex dose is 5 Gy. Assuming that the kinetics on monkey 2 apply, the blood dose is 3 Gy and the renal cortex dose is 3 Gy. Under the second assumption, that ²²¹Fr and ²¹⁷At decay with the ²¹³Bi, using kinetics of monkey 1, the blood dose is 3 Gy and the renal cortex dose is 13 Gy; using kinetics of monkey 2, the blood dose is 2 Gy and the renal cortex dose is 9 Gy.

View larger version (20K): [in this window] [in a new window]   FIGURE 8. (A and B) Relationship between 213Bi and 221Fr activity in whole blood. Recorded activities of 213Bi () and 221Fr () in monkey 1 (A) and monkey 2 (B) are shown. Each individual small curve represents data from single day (days 1, 5, 8, 14, and 18, respectively, after 225Ac-HuM195 administration). For each curve, x-axis is linear with respect to time during which isotope equilibrium is reached ex vivo. (C) Blood time–activity curves of 213Bi. Activity concentration for 213Bi was estimated by back-extrapolating 213Bi counts to time of blood collection. Dashed line (monkey 1) corresponds to 1-component exponential fit with intercept and t1/2 of 49 percentage injected dose per kilogram (%ID/kg) and 9 ± 2 d, respectively. Corresponding values for monkey 2 (solid line) are 34 %ID/kg and 11 ± 3 d, respectively.

	DISCUSSION

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²²⁵Ac labeling of HuM195 did not affect its immunoreactivity nor did labeling change the blood t_1/2 when compared with previous studies in monkeys using HuM195 (18). In addition, the chelated ²²⁵Ac is stable in vivo. This suggests that metabolism will be determined primarily by the carrier IgG and the physical properties of the radioisotopes. The blood t_1/2 of a monoclonal antibody is affected by specific recognition by the immune system or by binding to Fc receptors, thereby increasing clearance, and by specific binding to target cells. Monkeys do not express CD33. Therefore, HuM195 blood t_1/2 in monkeys is long (12 d), whereas in patients displaying CD33 binding sites on both normal monocytes and leukemia cells it is far shorter (2–3 d) (19,20). In humans, after binding sites become saturated or leukemia cells are cytoreduced, blood t_1/2 increased (18). The biologic t_1/2 of monoclonal antibody and the physical half-live of the ²²⁵Ac radioisotope are matched well. In contrast, the more commonly used short-lived -emitting isotopes ²¹³Bi (t_1/2 = 46 min) and ²¹¹At (t_1/2 = 7 h) demand more injections to maintain high blood activity concentrations. This results in a more complicated and expensive regimen and as well in accumulation of unlabeled, potentially blocking monoclonal antibodies over the course of the treatment.

Due to the short pathlength of -radiation, unbound -immunoconjugates circulating in blood are unlikely to cause significant toxicity. After the initial decay of ²²⁵Ac to ²²¹Fr, however, this daughter -emitting element, and the subsequent -emitting daughters, ²¹⁷At and ²¹³Bi, would be released. The distributions of these 3 -emitting daughter nuclides of ²²⁵Ac are no longer determined by the carrier molecule and thus may increase toxicity as compared with drugs carrying a single -particle–emitting isotope. Therefore, estimating appropriate doses of ²²⁵Ac from chelated ²¹³Bi or chemically conjugated ²¹¹At data (both of which have been used in humans) might not be accurate. Difficulties may arise when estimating the impact of free ²²¹Fr, ²¹⁷At, and ²¹³Bi for therapeutic efficacy or for toxicity, as there are no previous data available for the first 2 elements, and minimal data for bismuth. Studies with longer-lived bismuth isotopes show that a considerable fraction of bismuth will be accumulated and excreted in the kidneys (21). Thus, renal toxicity in this study was an expected finding. The data also suggest that toxicity will be determined to a major extent by the redistribution of ²²⁵Ac daughters. Nearly half of the last -emitting daughter nuclide (²¹³Bi) was cleared from the blood, indicating that redistribution of daughter nuclides occurred. Excretion of heavy metals can be enhanced by administration of chelators such as 2,3-dimercapto-1-propanesulfonic acid, ethylenediaminetetraacetic acid, DTPA, and dimercaptosuccinic acid (22). Therefore, similar methods might be beneficial to reduce toxicity.

In the 2 monkeys showing acute radiation nephritis, the estimated absorbed dose to the renal cortex (assuming localization of ²²¹Fr and subsequent daughters to the renal cortex) was 9–13 Gy. The renal cortex was chosen as the primary target tissue since there is evidence that heavy metals localize to this portion of the kidney (14). It is important to note that the absorbed-dose estimates are based on assuming a uniform deposition of -particle energy throughout the renal cortex mass. As with almost all absorbed-dose calculations, this is a necessary oversimplification in the absence of detailed microbiodistribution data. The implications and possible errors associated with such an oversimplification, however, are profound for -particle emitters due to their high potency and short range. Depending on the actual microbiodistribution, the biologic effects could be substantially greater or smaller than one might expect from an estimated absorbed dose of 13 Gy. In humans, the tolerated dose of external beam radiation yielding radiation nephritis in 50% of humans within 5 y (TD_50/5) is 28 Gy (17). Assuming that a relative biologic effectiveness (RBE) factor of 5 applies, the RBE-adjusted absorbed-dose range of 45–65 Gy to the monkey renal cortex is consistent with the toxicity observed in the monkeys.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
In this study we showed that ²²⁵Ac-HuM195 can safely be administered at a dose level of 28 kBq/kg, whereas dose escalation caused significant toxicity at cumulative doses of 215–370 kBq/kg. For repeated treatment cycles, pharmacokinetics did not appear to be influenced by an immune response against HuM195. However, antibody production against xenogeneic monoclonal antibodies can occur at any time during the treatment. Significant human antibody response to HuM195 has not been reported. In humans, the biologic t_1/2 of HuM195 is 4–5 times shorter than that in monkeys because of rapid targeting of the antibody-based drug and subsequent cellular internalization of the immune complex. Therefore, we assume that a dose level of at least 28 kBq/kg may be administered with tolerable acute side effects.

	ACKNOWLEDGMENTS

 
We thank Krista LaPerle, VMD, PhD, and Hai Nguyen, VMD, for careful review and valuable discussion of histopathology. We also thank Laike Stewart, VMD, and Felix Homberger, VMD, PhD, for helpful advice. The help from Ron Finn, PhD, Jing Qiao, Michael Curcio, and Catalina Cabassa for preparing the radiopharmaceuticals and George Gonzales and Donna Ortiz for excellent care of the animals is acknowledged. This work was supported by National Institutes of Health grants P01 33049 and R01 55349; the Steps for Breath Fund; William H. Goodwin and Alice Goodwin and the Commonwealth Cancer Foundation for Research and the Experimental Therapeutics Center of MSKCC; and the Doris Duke Charitable Foundation.

	FOOTNOTES

 
Received May 28, 2003; revision accepted Sep. 25, 2003.

For correspondence or reprints contact: David A. Scheinberg, MD, PhD, Department of Molecular Pharmacology and Chemistry, Memorial Sloan- Kettering Cancer Center, 1275 York Ave., New York, NY 10021.

E-mail: d-scheinberg{at}ski.mskcc.org

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 

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