In this study, we demonstrate that sequential administration of cytarabine and 213Bi-lintuzumab is tolerable and can produce remissions in some patients with AML. Although a relatively small group of heterogenous patients were included in this trial, it provides proof-of-principle that targeted α-particle immunotherapy may be effective at reducing low-volume disease. These results suggest that further investigation of radioimmunotherapy with α-emitters after cytoreduction and in the postremission or adjuvant setting is warranted.
Except as conditioning for HSCT, radioimmunotherapy with long-range, low energy β-emitters is useful only in treating bulky, radiosensitive cancers such as lymphoma, since DLT usually results from nonspecific irradiation of normal tissues. Conversely, therapy with high-energy, short-range α-particles can provide far more potent and selective delivery of radiation to individual tumor cells, yielding enhanced antitumor activity with decreased toxicity. Despite these advantages, CRs with 213Bi-lintuzumab alone would require extraordinarily high injected activities. If we assume tumor burdens of 1012 cells with an average CD33 density of 10,000/cell, approximately 1016 binding sites are available to lintuzumab. With the specific activities that are feasible, only 1 in 2,700 lintuzumab molecules carry the radiolabel. Therefore, it remains difficult to deliver 1–2 213Bi atoms to every leukemia cell, particularly because of its 46-minute physical half-life. In the setting of small-volume disease, however, the short path length and high LET of α-particles are ideal. In this study, cytoreduction provided by cytarabine allowed further reduction of residual leukemia by 213Bi-lintuzumab to produce remissions.
Although decreases in marrow blasts were seen at all dose levels, we observed a 213Bi dose-response relationship with remission occurring only at doses ≥ 37 MBq/kg. This suggests that cytarabine was not likely the sole cause of remissions. Moreover, serial bone marrow evaluations suggest that 213Bi-lintuzumab augmented the antileukemic activity of cytarabine alone. Although blasts were seen on all marrow samples after administration of cytarabine but before 213Bi-lintuzumab, these specimens were obtained before day 14 of therapy. Because of this early time point, it is impossible to determine whether any patient would have achieved a CR with cytarabine alone. The ability of sequential cytarabine and 213Bi-lintuzumab to induce remissions was seen only in patients where effective cytoreduction with cytarabine was possible. Six of 18 patients (33%) with untreated AML or untreated first relapse who received doses of ≥ 37 MBq/kg responded, while none of the 7 patients with primary refractory AML or multiply treated relapsed disease benefitted. This group of patients would not be expected to have a significant reduction in leukemic burden after single-agent cytarabine, and, as noted in an initial phase I study, treatment with 213Bi-lintuzumab alone at similar activities did not produce CRs in patients with large disease volumes.
The response rate in this trial was higher then expected from a single course of standard-dose cytarabine alone (31
). Bodey et al.
reported a 3% response rate after a single course of cytarabine at the dose and schedule used in this study (32
). Similarly, in a report by Bickers et al.
, no responses were seen after one course of cytarabine at doses of 200 mg/m2
/day for 5 days (33
). The response rate reported in these early trials may be lower than expected today because of significant improvements in supportive care over the past 30 years. There are numerous nonrandomized trials in the literature that report a response rate of 10–20% with low-dose cytarabine in older patients with AML, but, in general, multiple cycles of therapy are necessary. The largest randomized trial of low-dose cytarabine compared with hydroxyurea confirmed a response rate of 18%, but only after a median of three courses. One of 103 patients (1%) achieved CR after the first cycle of therapy (34
The current study demonstrates the impact of disease burden on antibody biodistribution. In the initial phase I study of single-agent 213
Bi-lintuzumab, in which similar total antibody doses were used, the percentage of injected activity reaching the marrow after multiple doses increased in 38% of patients, whereas activity in the liver and spleen decreased in 75% and 56% of patients, respectively (15
). This suggested that CD33 sites in the liver and spleen can act as antigen sinks and that as they become saturated, more drug reaches the marrow with repeated injections. Similarly, high numbers of circulating CD33-positive blasts or cell-free CD33 can adversely affect biodistribution of the drug by rapidly binding antibody and preventing it from reaching target sites within the marrow. In contrast, after partial cytoreduction in this trial, marrow activity remained constant or decreased in all 4 patients who were studied. Gamma camera imaging revealed cardiac blood pooling after the last injection of 213
Bi-lintuzumab in one patient, suggesting that saturation of all antigen sites was possible in patients with smaller disease burdens. Additionally, parametric rate imaging following the last injection of 213
Bi-lintuzumab showed decreased uptake or clearance of drug when compared to the first injection in all patients. Taken together, these data indicate greater saturation of antigen sites by 213
Bi-lintuzumab in target organs after partial cytoreduction than with 213
The strategy of arming lintuzumab with an α particle-emitting radionuclide was originally proposed to increase the modest immunologically-mediated antileukemic effects of the antibody itself. Based on a pilot study in which one of 10 patients with relapsed or refractory AML achieved a CR lasting over five years (12
) and a phase II study that confirmed a 6% response rate (13
), the role of lintuzumab in cytoreduced disease was examined in a randomized phase III trial (35
). Patients with relapsed or refractory AML received mitoxantrone, etoposide, and cytarabine alone or with lintuzumab. While an improvement in response rate attributable to unconjugated antibody therapy did not reach statistical significance (36% v
= 0.28), no difference in adverse events or treatment-related mortality between the two groups was seen. A more recent phase I trial was conducted to determine whether higher concentrations of lintuzumab sustained over prolonged periods could result in greater therapeutic efficacy (14
). In this study, 7 of 17 patients with AML responded. These results have led to an ongoing randomized phase II study of low-dose cytarabine with or without lintuzumab in older patients with untreated AML who are unable to tolerate standard induction chemotherapy.
Gemtuzumab ozogamicin (GO) represents an alternative antibody-based treatment to this radioimmunotherapeutic approach. GO agent is composed of a humanized anti-CD33 monoclonal antibody conjugated to a derivative of the potent antitumor antibiotic calicheamicin. When released from the immunoconjugate within the cytoplasm of a leukemic cell, calicheamicin induces DNA damage and subsequent apoptotic cell death. In a series of trials conducted in adults with AML in first relapse, a response rate (CR + CRp) of 26% was achieved (36
Typically, significant myelosuppression is seen with GO, even as a single agent. The median time to neutrophil recovery ≥ 1500/μL in responding patients was 48 days (36
). In contrast, resolution of grade 4 leukopenia occurred after a median of 22 days following administration of single-agent 213
Bi-lintuzumab in an earlier phase I study (15
). Following sequential therapy with cytarabine and 213
Bi-lintuzumab, responding patients in the current trial had neutrophil recovery after a median of 30 days. Treatment with GO is also associated with significant liver function abnormalities. Grade 3 or 4 hyperbilirubinemia and transaminase elevations were reported in 29% and 18% of patients, respectively. Sinusoidal obstructive syndrome was seen in 5% of patients (36
). Grade 3 or 4 liver function abnormalities, however, did not occur with single-agent 213
). When given after cytarabine in the current study, only 16% of patients developed significant hyperbilirubinemia, and 3% had grade 3/4 transaminase elevations. Sinusoidal obstructive syndrome was not observed. The more favorable toxicity profile of 213
Bi-lintuzumab suggests that integration of targeted α-particle immunotherapy into treatment strategies with standard chemotherapy may be more feasible than chemotherapy-GO combinations.
The ability of 213
Bi-lintuzumab to produce remissions in some patients with poor-risk AML in this trial provide the rationale for the use of α-particle immunotherapy in the setting of small-volume leukemias and cancers, or micrometastatic disease. The major obstacles to the widespread use of radioimmunotherapy with 213
Bi, however, are its short half-life and the requirement of an on-site 225
Bi generator. Therefore, we developed a second generation construct in which the isotope generator is directly conjugated to a tumor-specific antibody. In this strategy, 225
= 10 days) can serve as an in vivo
generator of 4 α-particles at or within a cancer cell. Based on the activity of 225
Ac-containing radioimmunoconjugates in several xenograft models (37
), we are currently conducting a phase I trial of 225
Ac-lintuzumab in advanced myeloid leukemia. Additional studies combining 225
Ac-lintuzumab with cytoreductive chemotherapy are planned.