Anti-CD45 Radioimmunotherapy with 90Y but Not 177Lu Is Effective Treatment in a Syngeneic Murine Leukemia Model

Radioimmunotherapy (RIT) for treatment of hematologic malignancies has primarily employed monoclonal antibodies (Ab) labeled with 131I or 90Y which have limitations, and alternative radionuclides are needed to facilitate wider adoption of RIT. We therefore compared the relative therapeutic efficacy and toxicity of anti-CD45 RIT employing 90Y and 177Lu in a syngeneic, disseminated murine myeloid leukemia (B6SJLF1/J) model. Biodistribution studies showed that both 90Y- and 177Lu-anti-murine CD45 Ab conjugates (DOTA-30F11) targeted hematologic tissues, as at 24 hours 48.8±21.2 and 156±14.6% injected dose per gram of tissue (% ID/g) of 90Y-DOTA-30F11 and 54.2±9.5 and 199±11.7% ID/g of 177Lu-DOTA-30F11 accumulated in bone marrow (BM) and spleen, respectively. However, 90Y-DOTA-30F11 RIT demonstrated a dose-dependent survival benefit: 60% of mice treated with 300 µCi 90Y-DOTA-30F11 lived over 180 days after therapy, and mice treated with 100 µCi 90Y-DOTA-30F11 had a median survival 66 days. 90Y-anti-CD45 RIT was associated with transient, mild myelotoxicity without hepatic or renal toxicity. Conversely, 177Lu- anti-CD45 RIT yielded no long-term survivors. Thus, 90Y was more effective than 177Lu for anti-CD45 RIT of AML in this murine leukemia model.


Introduction
Acute myeloid leukemia (AML) is associated with high rates of relapse and mortality and despite aggressive treatments such as hematopoietic cell transplantation (HCT) many patients fail to achieve long-term survival. Attempts to decrease relapse after HCT have, among other approaches, utilized intensified cytoreductive therapies either by increasing total body irradiation (TBI) or doses of chemotherapy during HCT conditioning. Escalated TBI doses for HCT preparative regimens have led to fewer relapses, but these efforts have typically not translated into improved overall survival (OS) because of increased treatmentrelated mortality [1][2][3]. In contrast, the use of radiolabeled monoclonal antibodies (Ab) directed at cell surface antigens allow for the targeted delivery of escalated doses of radiation to bone marrow (BM), spleen, and other sites of malignancy while sparing normal organs [4][5][6][7][8][9][10][11]. In addition, RIT may improve outcomes when used in combination with chemotherapy and/or HCT [10,[12][13][14]. Though leukemia cells express multiple surface antigens that could be targeted, clinical RIT trials to treat AML have primarily used anti-CD33, anti-CD66 and anti-CD45 Ab as vehicles to deliver radiotherapy. CD45 is present on more than 70% of nucleated cells in normal BM, and on more than 85% of leukemic samples [15][16][17], with an average copy number of ,200,000 molecules per cell [18].
The radionuclides employed in RIT to date have limitations. We have used iodine-131 ( 131 I) in our clinical and pre-clinical studies because there is extensive experience with its medical use, the technology for radiolabeling Abs with iodine is well established, and its gamma component allows direct determination of labeled Ab biodistribution. However, the high-energy gamma component of 131 I requires that patients be treated in radiation isolation, and poses a radiation exposure risk for staff and family. In addition, not all facilitates are capable of handling and disposing of 131 I waste. To supplant 131 I-anti-CD45 Ab an alternative radionuclide yttrium-90 ( 90 Y) has been selected as a therapeutic radioisotope for our studies because it is a pure b-emitter that is commercially available in high specific activity and purity. Moreover, 90 Y has a high-energy tissue penetration. However, 90 Y cannot be imaged directly for which an imaging surrogate for dosimetry studies is required for 90 Y. Therefore, a need remains for alternative radionuclides that can be used for imaging procedures, with adequate energy profiles to achieve therapeutic effects. Lutetium-177 ( 177 Lu) potentially fulfills this need as its beta-emission energy, path length, and half-life are similar to the efficacious 131 I. However, unlike 131 I, 177 Lu has lower and safer energy gamma-emissions that do not require isolation, and facilitate imaging for dosimetry. In addition, 177 Lu with a shorter path length (0.9 mm) offers the potential for less non-specific toxicity compared to 90 Y (path length 52.7 mm). We hypothesized that 177 Lu may be an efficacious alternative radionuclide to 90 Y for the treatment of hematologic malignancies with anti-CD45 RIT. In these studies we compared the therapeutic efficacy and toxicity of 177 Lu-and 90 Y-anti-CD45 RIT as primary treatment in an immunocompetent, syngeneic murine myeloid leukemia model, and showed that 90 Y was more effective than 177 Lu for anti-CD45 RIT of AML.

Mice
Female B6SJLF1/J mice (6 to 12 weeks old) were purchased from Jackson Laboratories (Bar Harbor, ME). Imaging studies used female athymic mice (6 to 12 weeks old) from Harlan Laboratories (Indianapolis, IN). Mice were housed at the FHCRC animal care facility in a pathogen-free environment, and handled by protocols approved by the FHCRC Institutional Animal Care and Use Committee (IACUC IR #1716). This study was carried out in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health, and all efforts were made to minimize suffering.

Cell Lines, Antibodies and Radiolabeling
Murine AML cells were produced as previously described by serial passage in SJL/J mice [19,20]. Control Ab (polyclonal rat IgG) was purchased from Sigma Aldrich (St. Louis, MO). Rat IgG2b anti-murine CD45 Ab (30F11) was purified from mouse ascites, and DOTA-30F11 and DOTA-rat IgG were prepared as previously described [21]. Yttrium-90 was purchased from PerkinElmer, Inc. (Waltham, MA), while 177 Lu was acquired from either PerkinElmer, Inc. or the University of Missouri Research Reactor (MURR, Columbia, MO). DOTA-30F11 and DOTArat IgG were radiolabeled with 177 Lu or 90 Y as previously described [22], and were PD10 column (Bio-Rad, Hercules, CA) purified, resulting in labeling efficiencies for all 90 Y-and 177 Lu-DOTA-Ab injectates of .90%. Radiochemical purity determined by HPLC was .99% for each radiolabeled DOTA-30F11 construct.

Biodistribution Studies
The percent injected dose of radioactivity per gram of tissue (% ID/g) for 90 Y-and 177 Lu-DOTA-30F11, with correction for radioactive decay, were performed in groups of five mice, as previously described [19].

Cerenkov Imaging
Imaging studies used female athymic mice from Harlan Laboratories (Indianapolis, IN). Radiolabeled DOTA-Ab (0.67 nmol) (100 mCi of either radionuclide) was delivered via tail vein to up to five athymic nude mice at time 50 hours. Mice were anesthetized and imaged with 2.5% isoflurane, using a Xenogen IVIS Spectrum platform from PerkinElmer, Inc. (Waltham, MA). Images were acquired with Living Image software (v 3.0, PerkinElmer, Inc.) at 0.25, 1, 2, 4, 24, and 48 hours after delivery of each radiolabeled Ab with camera settings of: medium binning, f-stop 1, open filter channel, and image acquisition length of 15 seconds ( 90 Y) or 2 minutes ( 177 Lu). The fluorescence intensity scale was determined using background-corrected fluorescent whole-body images measured automatically by the software.

Radioimmunotherapy of Leukemic Mice
RIT studies were performed in groups of 10 mice as previously described with 100 or 300 mCi 90 Y-or 177 Lu-DOTA-30F11, or with 300 mCi of either 90 Y-or 177 Lu-DOTA-rat IgG [19,20]. Briefly, mice were placed on a Uniprim-containing diet (irradiated, 4100 ppm; Animal Specialties, Hubbard, OR) 3 days before i.v. injection with 10 5 SJL leukemic cells. Two days later mice received radiolabeled DOTA-30F11 or DOTA-rat IgG. Mice were monitored daily after RIT injections, and weighed 2-3 times per week. Mice were sacrificed in a CO 2 chamber per IACUC protocols for .30% weight loss, for severe lethargy or if significantly moribund.

Radiation Dosimetry
Using biodistribution data and the standard Medical Internal Radiation Dose Methods [23], radiation absorbed doses for blood and each organ were calculated as described previously [24,25].

Toxicity Assessments
Groups of 10 non-leukemia bearing B6SJLF1/J mice were given 300 mCi 90 Y-DOTA-30F11, or 300 or 665 mCi 177 Lu-DOTA-30F11 by tail vein injection. Mice were bled via the retro-orbital plexus at baseline, 1, 2, 3, 4, and 8 weeks after injection of radiolabeled Ab. Five mice per group had blood analyzed for complete blood counts, and the other 5 mice had blood analyzed for kidney and liver function. Values were followed serially and compared to those from untreated age-matched control B6SJLF1/J mice.

Biostatistics
Comparisons were made among groups of 5 mice or 10 mice. Five mice per group provide 80% power to observe a statistically significant difference (at the 2-sided significance level of.05) in a continuous outcome if the true difference between groups is 2.02 standard-deviation units; 10 mice per group provide 80% power if the true difference is 1.33 standard-deviation units. The two-sample t-test was used to compare continuous outcomes, and the log-rank test was used to compare survival between groups. In addition, 95% confidence intervals for the difference between groups were given when comparing the difference between groups.

Comparative Biodistribution Studies
The biodistribution of 90 Y-or 177 Lu-radiolabeled anti-CD45 antibodies (DOTA-30F11) were compared in mice harboring syngeneic murine myeloid leukemia. B6SJLF1/J mice were given 10 5 myeloid leukemia cells via tail vein injection and two days later 90 Y-or 177 Lu-DOTA-30F11 was administered. Mice were euthanized after 6, 24, or 48 hours and their organs were harvested and analyzed on a gamma counter to calculate the % ID/g. Biodistribution studies demonstrated excellent localization of both 90 Y and 177 Lu to the BM and spleen, with much less uptake in non-target (i.e., non-hematolymphoid) organs. Although the majority of 177 Lu-DOTA-30F11 Ab conjugate rapidly localized to spleen (158.7¡12.2% ID/g) and BM (54.7¡21.5% ID/g) after 6 hours, significant uptake of 177 Lu-DOTA-30F11 was also noted in blood-rich organs such as lung (23.7¡3.9% ID/g), liver (23.3¡3.6% ID/g), and kidney (31.3¡3.7% ID/g) at 6 hours. Less 90 Y-DOTA-30F11 was present in non-target organs at 6 hours (lung: . These biodistribution data suggest that both radionuclides were effectively targeted to BM and spleen with minor uptake in non-hematologic organs. To underscore the preferential targeting of hematologic organs compared to normal organs, ratios of the % ID/g of target organs (BM and spleen) to normal organs were calculated for both 177 Lu-and 90 Y-radiolabeled Ab after 24 and 48 hours. Target-to-normal organ ratios for dose-limiting organs were higher but not statistically significant for 90 Y-DOTA-30F11 than for 177 Lu-DOTA-30F11 after 24 hr. with an average BM-to-lung ratio of 5.6:1 compared to 3.5:1 for 177 Lu (p50.1747), and BM-to-liver ratios of 4.7:1 and 2.8:1 for 90 Y and 177 Lu (p50.0969), respectively ( Fig. 2A). These ratios were stable or improved with time, presumably due to progressive blood clearance (Fig. 2B). Spleen-to-normal organ ratios were higher than BM-to-normal organ ratios for both 90 Y and 177 Lu ( Fig. 2C and D), likely due to the higher density of CD45 + cells in the spleen in this leukemia model. Radiation-sensitive normal organs were relatively spared from excessive radiation exposure, with spleen-to-lung ratios of 18.0:1 and 13.3:1 after 24 hours for 90 Y and 177 Lu, respectively, and spleen-to-liver ratios of 15.3:1 and 9.5:1 for 90 Y and 177 Lu, respectively. Target-to-normal organ ratios improved 48 hours after radiolabeled Ab delivery, with spleen-to-lung ratios of 33.5:1 and 36.1:1 and spleen-to-liver ratios of 25.8:1 and 19.9:1 for 90 Y-DOTA-30F11 and 177 Lu-DOTA-30F11, respectively. In addition, the kidneys, a particular source of concern in radioimmunotherapy studies, showed favorable radiation exposure with BM-to-kidney ratios of 3.0:1 and 3.1:1 after 48 hours for 90 Y and 177 Lu, respectively, and spleen-to-kidney ratios of 20.6:1 and 19.2:1 for 90 Y and 177 Lu, respectively.
Comparative in vivo Cerenkov Light Imaging of 90 Y-and 177 Lu-DOTA-30F11 We sought to visualize the in vivo targeting of 90 Y-and 177 Lu-DOTA-30F11 via Cerenkov Light Imaging (CLI) experiments. Cerenkov radiation arises when charged particles such as b 2 , or b + emissions travel through an optically transparent insulating material (typically water) with a velocity that exceeds the speed of light. As charged particles travel through water, they lose kinetic energy by polarizing the electrons of water molecules. Relaxation of the polarized molecules occurs via the emission of light energy, giving rise to the observed Cerenkov emission, or a continuous spectrum of light from near-ultraviolet to visible [26][27][28][29]. Athymic nude mice were injected with 300 mCi of either 90 Y-or 177 Lu-DOTA-30F11, and both radiolabeled Ab conjugates displayed distinct, focal localization to the spleen by CLI as early as 15 minutes post-injection. The spleens were the major organ of localization in all imaged mice, and the overwhelming splenic pixel intensity could have prevented visualization of other sites of uptake. As expected, imaging confirmed increased uptake in CD45 + organs, especially spleen and liver, for both radionuclides. However, 90 Y-DOTA-30F11 treated mice had higher signals (peak radiance of 1.2610 6 p/sec/cm 2 /sr) than 177 Lu-treated mice (peak radiance of 6.8610 3 p/sec/cm 2 /sr), likely because of the higher decay energy and longer path length of 90 Y compared to 177 Lu (Fig. 3).
Radioimmunotherapy with 90 Y-DOTA-30F11 versus 177 Lu-DOTA-30F11 As both radionuclides effectively targeted CD45 + tissues in vivo we then tested 90 Y-and 177 Lu-DOTA-30F11 to treat disseminated syngeneic myeloid leukemia in mice. Ten mice per group were given 10 5 SJL leukemic cells via tail vein injection and two days later were treated with 300 or 100 mCi of either 90 Y-or 177 Lu-DOTA-30F11. A prior pilot study demonstrated that 300 mCi of 90 Y had previously been well tolerated by mice from a related SJL strain [19]. All ten untreated control mice died, with the median survival (the time at which the Kaplan-Meier estimate of survival reaches 50% or lower) being 39 days. Eight of the 10 mice that received 100 mCi 90 Y-DOTA-30F11 died, with the median survival being 66 days, and 4 of the 10 mice that receive 300 mCi 90 Y-DOTA-30F11 died with the median survival not reached (Fig 4A). These data showed a statistically significant trend in decreased mortality as dose increased (from control to 100 mCi to 300 mCi, p.003). On the other hand, all 10 mice that received 100 mCi 177 Lu-DOTA-30F11 died as did all 10 mice that received 300 mCi 177 Lu-DOTA-30F11 (median day of survival, 52 days, 16 days, respectively). There was actually a statistically significant trend for increased mortality as the dose of 177 Lu-DOTA-30F11 increased. To confirm that the therapeutic benefits observed were due to targeting of radionuclides to the BM and spleen and not to a nonspecific radiation effect, mice were also treated with an isotype negative control DOTA-rat IgG that was radiolabeled with 300 mCi of either 90 Y or 177 Lu (Fig. 4B). Therapy with 300 mCi 90 Y-DOTA-rat IgG resulted in excessive toxicity with 80% (8/10) of mice requiring euthanasia for excessive weight loss, contributing to a median survival of 9 days after therapy. Mice treated with 300 mCi 177 Lu-DOTArat IgG had a median survival of 45 days, with the deaths from progressive leukemia characterized by splenic enlargement from leukemic infiltration. These results suggest that 90 Y-DOTA-30F11 improved survival in this leukemia model in a dose-dependent manner, while 177 Lu-DOTA-30F11 was too toxic or ineffective at similar doses.

Dosimetry Estimates of Absorbed Doses of Radiation Delivered using 90 Y and 177 Lu
The absence of a therapeutic benefit when using 177 Lu-DOTA-30F11 despite similar targeting specificity of 90 Y-DOTA-30F11 was unexpected. We hypothesized this might be due, at least in part, to differences between absorbed radiation doses in target organs such as the BM and spleen. Consequently, we estimated the absorbed radiation doses for organs of interest per unit of activity injected for both 90 Y-and 177 Lu-DOTA-30F11 [23]. Dosimetry calculations showed that the BM absorbed dose per mCi of 90 Y-DOTA-30F11 injected was more than 11-times greater than the absorbed radiation dose delivered by 177 Lu-DOTA-30F11 (8.5 versus 0.74 cGy/mCi; Table 1). Although not as dramatic, the spleen absorbed dose per unit injected was more than 2-fold higher for 90 Y-than 177 Lu-DOTA-30F11 (82.5 versus 31.9 cGy/mCi). All other organs from mice treated with 90 Y-DOTA-30F11 had 2-to 4-fold increases in absorbed dose per unit injected compared to mice treated with 177 Lu-DOTA-30F11. The markedly disparate absorbed doses between 90 Y and 177 Lu based anti-CD45 RIT could be due to the higher energy profile of 90 Y (E max 52.3 MeV) compared to 177 Lu

Assessment of Toxicity after RIT using 90 Y-versus 177 Lu-DOTA-30F11
To evaluate the tolerability of 90 Y-and 177 Lu-anti-CD45 RIT, mice were given 300 mCi 90 Y-or 177 Lu-DOTA-30F11. Because these injected doses do not yield comparable absorbed radiation doses in dose-limiting organs, an equivalent liver absorbed dose was also used in toxicity studies (665 mCi 177 Lu-DOTA-30F11, the equivalent liver absorbed dose to 300 mCi 90 Y-DOTA-30F11). Laboratory studies were performed before the start of therapy, 1, 2, 3, 4 and 8 weeks after injection of each radiolabeled DOTA-30F11 conjugate. Blood was analyzed for complete blood counts and renal and hepatic functions. The most pronounced toxicity detected with both radionuclides was myelosuppression with minimal impact on renal or hepatic functions. The baseline white blood cell count (WBC) in untreated control animals was 7.3¡0.8 K/mL. Mice treated with anti-CD45 RIT exhibited an expected leucopenia 1 week after injection of radiolabeled DOTA-30F11 with a WBC nadir of 1.7¡1.9 (23.3% of untreated controls), 0.3¡0.1 (4.1% of untreated controls), and 0.1¡0.03 K/mL (1.4% of untreated controls) for mice treated with 300 mCi 90 Y-, 300 mCi 177 Lu-, and 665 mCi 177 Lu-DOTA-30F11, respectively (p50.5046; Fig. 5A). Mice treated with 665 mCi 177 Lu-DOTA-30F11 developed fatal anemia and thrombocytopenia as these mice died by week 2 after RIT injections [mean hemoglobin (Hb) 2.6¡1.3 g/dL (16.3% of untreated controls), mean platelet count 7.3¡1.8 K/mL (0.7% of untreated controls), compared to mean Hb 16.0¡1.5 g/dL [difference of 13.  Additional experiments showed minimal renal or hepatic toxicity from 90 Yand 177 Lu-anti-CD45 RIT in this model. Serum Cr and BUN values remained within normal limits throughout the study (,0.5 mg/dL and 10-36 mg/dL, respectively; Fig. 5D and E). Mice treated with 665 mCi of 177 Lu-DOTA-30F11 had higher BUN levels (mean 34¡5.6 mg/dL) at week 2, with a difference of 12 mg/dL ([5.6-18.4], p50.0039) compared to untreated control mice, likely from gastrointestinal bleeding. Minimal hepatic dysfunction was detected after RIT with either radionuclide, although mice treated with 665 mCi of 177 Lu-DOTA-30F11 exhibited slightly increased AST levels at week 2 after therapy (mean 147.7¡20.2 U/L), compared to the mice treated with 300 mCi of 90 Y [mean AST 55.0¡14.9; for a difference of 85.9 U/L ([37.5-134.2], p50.0049), Fig. 5G]. Other liver function tests, such as ALT (Fig. 5F), and ALP (Fig. 5H) from RIT treated mice did not differ from untreated matched control mice throughout the 8 weeks of monitoring. In summary, toxicity studies showed transient myelotoxicity with anti-CD45 RIT that normalized by 4 weeks after therapy, with minimal hepatic or renal toxicity.

Discussion
The results suggest that 90 Y-anti-CD45 RIT was more effective than 177 Lu-anti-CD45 RIT to treat leukemia in a syngeneic murine model (Fig. 4). Biodistribution studies showed both 90 Y-and 177 Lu-DOTA-30F11 localized similarly to target sites with the highest disease burden, BM and spleen (Fig. 1). In vivo Cerenkov imaging (Fig. 3) allowed visualization of radiolabeled DOTA-30F11 with both b - emitting radionuclides localizing to the spleens, confirming specific targeting by biodistribution studies. Furthermore, anti-CD45 RIT using 300 mCi 90 Y or 177 Lu was well tolerated, since the most pronounced toxicity was transient myelosuppression without renal or hepatic dysfunction (Fig. 5). Thus, the lack of efficacy from 177 Lu-DOTA-30F11 was not from an inability to deliver 177 Lu to sites of disease, but may be explained by their differences in physical energy properties. Residence time of 90 Y-or 177 Lu-DOTA-30F11 should not have varied, as residence time is more dependent on the stability of the bioconjugate than on the specific radionuclide. Treatment with 177 Lu should have concentrated the decay energies closer to target tissues than 90 Y given the shorter path length of 177 Lu (0.9 mm) compared to that of 90 Y (2.7 mm), according to theoretical calculations. If we consider absorption energies in hypothetical spheres of 0.1, 1 and 10 mm diameter, because of the effective path length differences, the fraction of energy deposited in these spheres would be 1, 9, and 66% for 90 Y compared to 15, 67, and 97% for 177 Lu, respectively [30,31]. Unfortunately, their decay energies are significantly different such that even if this schema was followed the absolute energy differences may not have allowed for comparable radiation doses. 177 Lu has a low energy gamma-radiation fraction (0.2 MeV) with the majority in the beta range (78.6% at 0.5 MeV) and a mean energy beta-radiation of E mean 50.13 MeV [32]. For comparison, the mean energy from 131 I beta-radiation is about E mean 50.18 MeV while the E mean of 90 Y50.9 MeV [31]. Taken together, the mean beta-particle energy per decay emitted by 90 Y appears to be 7-fold higher than that of 177 Lu, resulting in lower effective radiation doses to target tissues. Dosimetry calculations support this hypothesis as absorbed doses in tissues from mice treated with 90 Y-DOTA-30F11 were 2-to 11-fold higher than mice treated with 177 Lu-DOTA-30F11 (Table 1). The efficacy differences between 90 Y-and 177 Lu-DOTA-30F11 may be explained, at least partially, by the large differences in decay energy that lead to disparate absorbed doses of the two radionuclides. Absorbed doses were lower in 177 Lu-DOTA-30F11 treated mice compared to mice treated with 90 Y-DOTA-30F11 when each group received 300 mCi of radioactivity. However, mice given an equivalent liver absorbed dose (665 mCi of 177 Lu-DOTA-30F11) had fatal hematologic toxicities, suggesting that absorbed dose alone may not explain the differential efficacy. Mice treated with 665 mCi 177 Lu-DOTA-30F11 received 212 Gy of radiation delivered to the spleen compared to a similar dose of 248 Gy delivered to the spleen when treated with 300 mCi 90 Y-DOTA-30F11. Consequently, a correlate explanation for the efficacies may lie in dose rate differences; at the spleen, 248 Gy from 300 mCi of 90 Y-DOTA-30F11 was delivered at a higher dose-rate over a smaller time frame given the shorter half-life of 90 Y, compared to the 212 Gy from 665 mCi 177 Lu-DOTA-30F11 at a lower dose-rate over its longer half-life. Indeed, clinical trials have shown increased relapse rates with increased dose fractionation and reduced dose-rates [33][34][35]. Similarly, murine transplant models that increased dose fractionation or lowered dose-rate of radiation effectively restored host hematopoiesis, and required higher total TBI doses for donor engraftment [36], suggesting that lower dose-rates may significantly lower the cytotoxic effect of radiation.
Lastly, these results also assessed the potential long-term toxic effects of 90 Yversus 177 Lu-anti-CD45 RIT. Mice without leukemia used in toxicity studies may explain why mice given 300 mCi 177 Lu-DOTA-30F11 did not display the fatal toxicities that were seen in the RIT studies employing disease-bearing animals. While speculative, leukemic cells used for RIT experiments provided additional CD45 + target, which lead to more radioactivity in hematolymphoid tissues producing more hematologic toxicity. Further, myelotoxicity could have been more pronounced with 177 Lu because the longer half-life interfered with effective hematopoiesis, risking potentially fatal infections, lethal anemia, and/or bleeding complications.
In summary, these studies in a syngeneic disseminated leukemia model confirm the therapeutic efficacy of 90 Y-anti-CD45 RIT for leukemia but do not support the addition of 177 Lu to RIT treatment options. The lack of efficacy using 177 Lu was not due to suboptimal targeting as both radionuclides were delivered equally to BM and spleen, but may have been due to differences in radiation properties, such as decay energies, effective path-lengths, and dose-rates. The longer half-life of 177 Lu was unable to provide a comparable absorbed dose of 90 Y, and in fact its longer half-life and radiation effects may have interfered with effective hematopoiesis and further added to its myelotoxicity.