¹³¹I-tositumomab (originally called the “B-1” or “anti B-1” antibody) plus unlabeled tositumomab therapy of lymphoma was developed in a somewhat unconventional manner. The CD20 antigen, originally known as the B-1 antigen, was among the first pan B-cell marker antigen/antibody pairs (Stashenko et al. 1980; Tedder et al. 1985). A challenge with the CD20 target is that CD20 is present on normal B-cells as well as B-cell lymphomas (Fig. 1). However, an advantage of CD20 is that it is not present on pluripotent hematopoietic stem cells (Fig. 2). Pre-clinical studies showed that the I-131 anti-CD20 antibody,¹³¹I tositumomab, had greater anti-tumor effects than unlabeled tositumomab (Buchsbaum et al. 1992).

To deal with the dual challenges of circulating normal B-cells expressing the CD20 antigen, which would be the first cells to interact with the infused antibody, and the expected variability of clearance, a method was developed for patient dosing which addressed both challenges. The challenge with expected variability in clearance is that varying mCi doses would need to be infused to deliver the same radiation-absorbed dose (Fig. 3). Only by knowing if patients are fast or slow clearers of the radioantibody could the proper mCi amount of activity to infuse be determined.

In the phase I study, varying masses of unlabeled antibody pre-dosing were evaluated in an intra-patient comparison, coupled with individualized patient and tumor dosimetry. Thus, a tracer dose of radioantibody, the “dosimetric dose,” was given, and then daily gamma camera/or probe counts were obtained to determine the rate of clearance of the radioactivity from the patient (Kaminski et al. 1993).

In individual patients, varying masses of unlabeled monoclonal antibody were given before up to three separate dosimetric doses, up to three different mass levels (including no unlabeled antibody pre-dose). A simplifying assumption was made that the radioantibody was distributed rather uniformly in the patient, and a dosimetric calculation was made to determine the appropriate dose for a therapeutic effect. The dose escalation was performed guided by the projected total body radiation-absorbed dose with the maximum tolerated dose (MTD) being hematopoietic toxicity. An overview of the final Bexxar treatment regimen is shown in Fig. 4.

As shown in Fig. 5, a patient receiving no unlabeled antibody pre-dose on the baseline study and a large mass of unlabeled antibody pre-dose on a follow-up study, had significant effects on the in vivo biodistribution of the radioantibody. Low anti-CD20 mass pre-dose (or no protein mass pre-dose) was associated with rapid clearance of the radioantibody from the blood and a lower tumor radiation absorbed dose than when the radioantibody dosing was preceded by unlabeled anti-CD20 antibody. Higher protein masses were associated with a higher area under the curve (AUC) in the blood (Fig. 6).

Tumor dosimetry in this phase I study showed no decrement in tumor targeting relative to the total body-predicted radiation-absorbed dose at higher unlabeled tositumomab infusion mass levels. While the total body clearance of the dosimetric dose was slower at higher protein mass levels, there was no clear blocking of tumor targeting of the radioantibody.

Higher mass doses of tositumomab by itself were expected to have higher anti-tumor activity due to the known immunological effects of the unlabeled antibody (Buchsbaum et al. 1992). Thus, the protein mass for the therapeutic regimen was selected to be 450 mg, the highest mass used in the phase I study. It is notable, however, that this determination was made in patients who had not received prior rituximab therapy and who had rather substantial tumor burdens.

It is possible that the settled up, 450 mg, unlabeled anti-CD20 pre-dose could be too high for patients with low tumor burdens or who have had recent rituximab therapy. It is clear by basic immunological principles that unlabeled antibodies will ultimately be able to block the targeting of radiolabeled antibodies to tumor. The pre-dose of unlabeled tositumomab effectively blocked the uptake of the dosimetric dose of ¹³¹I-tositumomab to circulating B-Cells and to the spleen and there has been some evidence of tumor blocking in animal models (Gopal et al. 2008).

The other challenge in using an antibody which cross reacts with normal tissues was that the biodistribution might not be consistent depending on the amount of antibody given, the mass of tumor cells, and the number of CD20 positive cells accessible from the blood. The phase I dose escalation was performed based on patient-specific dosimetry, in which the total body radiation dose was increased in 10 cGy increments, as predicted by the dosimetric dose. The total mCi administered was driven by patient-specific dosimetry and not by mCi/kg or simply mCi. This showed that the maximum tolerated dose (MTD) was approximately 75 mCi in heavily pretreated patients who had not received rituximab therapy (Fig. 7). In the early report of the phase I study, safety was encouraging as was anti-tumor response, with 6/9 treated patients responding and four having complete responses, which was supported as the study was expanded to MTD (Kaminski et al. 1993, 2000; Wahl et al. 1998b).

There is a great deal of discussion regarding personalized cancer therapies and the use of imaging to guide therapies. ¹³¹I-tositumomab is a personalized therapy on at least two levels: (1) the molecular target, CD20, must specifically be present on the tumor for the treatment to be used and, (2) the administered dose is individualized based on the patient-specific dosimetric imaging.

The dosimetry and radiation safety aspects of ¹³¹I-tositumomab + unlabeled tositumomab therapy have been discussed in detail elsewhere (Gates et al. 1998). In the US, under current NRC rules, the therapeutic regimen can be given in the outpatient setting in nearly all cases, with close attention to radiation safety (Gates et al. 1998). It is important to note that clinical trial data from several studies have shown there to be considerable variability in the total body clearance rates of tracer doses of ¹³¹I-tositumomab. This results in considerable variability (when coupled with differences in body habitus) in the mCi required to deliver the targeted 75 cGy total body dose that is appropriate in most NHL patients with platelet counts >150,000 mm³ (Fig. 8).

In the FDA-approved version of the drug (see clinical data below), a simplified dosimetry scheme in which a limited number of data points are used has been employed (Fig. 4). In brief, the clearance of ¹³¹I-tostitumomab from the whole body is monoexponential, in general, and the clearance can be represented well by just 3 time points of imaging (or probe counts). We prefer gamma camera imaging to secure information on biodistribution, but early clinical trials were done and included a whole body NaI probe system focused at the central body, securing kinetic data.

By knowing the patient’s weight, by determining a maximum effective mass (to minimize overdosing of obese patients based on weight), it was possible to develop tables of activity-hours (mCi-hrs) required to deliver a given radiation dose to a patient. These values are predicted by the dosimetry scans. The activity-hours to deliver 75 cGy are noted in Fig. 9. A very simple calculation then follows to determine the therapeutic mCi dose for a specified total body dose level (Fig. 10). The FDA-approved drug, Bexxar, depends on use of this dosimetry approach, which has been discussed in several reports (Kaminski et al. 1992, 1993; Wahl et al. 1998a). This dosimetry approach also works well for ¹³¹I-rituximab therapy, which has been deployed in Australia (Calais and Turner 2012).

In clinical trials investigating ¹³¹I-tositumomab in patients with recurrent NHL, including both follicular recurrences, or transformed lymphomas, overall response rates have ranged from 60–83 %, with complete response (CR) rates of 15–52 %. The median duration of response with RIT ranges from 6–16 months for all responders, but seems to be associated with the quality of the initial response. At the time of publication of several studies, the median duration of response for patients who achieved a CR was not yet reached. It was very clear from the early studies that achieving a CR resulted in a high probability of long-term disease free status (Kaminski et al. 2000, 2001); Vose et al. 2000).

Higher response rates have been reported for RIT compared to the unlabeled antibody. In a cross-over study of ¹³¹I-tositumomab, 42 % of patients nonresponsive to unlabeled tositumomab achieved a CR after receiving a therapeutic dose of ¹³¹I-tositumomab (Davis et al. 2004). This supports the value of the radioactive component of the therapy, though it also shows there is anti-tumor activity for the unlabeled tositumomab. A partial explanation for the improved overall response rates with the radioactive monoclonal antibodies is that the addition of the radioactivity probably allows more cells to be targeted and killed due to the radiation cross-fire effect.

Although the initial results of the studies investigating ¹³¹I-tositumomab in recurrent NLH (mostly follicular) were excellent, the studies preceded those of rituximab. These trials included mostly patients who were rituximab naϊve and studies were needed to show efficacy of RIT in the group of patients previously treated with rituximab, which is now a routine component of B-cell lymphoma regimens.

Horning et al. described the therapeutic efficacy of whole body doses of 65–75 cGy of ¹³¹I-tositumomab (Bexxar) in 40 patients (24 rituximab nonresponders, 11 < 6 month responders, and 5 > 6 month responders) (Horning et al. 2005). In addition, the population included 59 % who were chemotherapy resistant. This group of patients had been extensively pretreated, with a median of four prior types of treatment before radioimmunotherapy. The overall response rate in this difficult to treat group was 65 %, with a CR rate of 38 %. Responses were not related to the prior response to rituximab. The median PFS was 24.5 months for responders and was not yet reached for those patients achieving a CR—again supporting the durability of a CR in response to the Bexxar regimen. Of particular note was an 86 % response rate (57 % CR) in patients with low-grade follicular lymphomas and tumors <7 cm in diameter. Toxicities were typically hematological, with grade 3 or 4 marrow toxicity in 50 % of cases. These strong data were instrumental in the FDA approval of the drug and its labeled indication (Horning et al. 2005).

Numerous prognostic factors have been evaluated in an attempt to identify which patients are most likely to respond to RIT. In individual studies, factors reported to be associated with response are low-grade follicular histology, nonbulky disease, no bone marrow involvement, fewer prior regimens, normal serum lactate dehydrogenase levels, a lower fractional prognostic index (IPI), a higher total body radiation dose, higher tumor radiation dose, and no prior history of transplants (Kaminski et al. 2000, 2001; Vose et al. 2000; Davis et al. 2004; Horning et al. 2005). Of all of these, low-grade follicular histology and nonbulky disease at the time of treatment are the most reliable predictors, as was well shown in the study by Horning et al. (2005).

In patients with NHL, RIT is generally well tolerated. The most common adverse events are serious cytopenias and these are experienced by a majority of patients. 60–70 % of patients enrolled in clinical trials developed grade III/IV thrombocytopenia and/or neutropenia. Count nadirs typically occurred between 4–7 weeks post-RIT and cytopenias may last for approximately 30 days. Consequently, monitoring of complete blood counts is required for at least 10–12 weeks post-therapy. Hematologic support is required for ∼15–32 % of patients after RIT (Kaminski et al. 2000, 2001; Vose et al. 2000; Davis et al. 2004; Horning et al. 2005).

Additional nonhematologic short-term side effects are usually mild and may include fatigue, asthenia, nausea, and fever. There are several long-term side effects that prescribing nuclear medicine physicians/radiologists, oncologists, and patients must also be aware (Kaminski et al. 2000, 2001; Vose et al. 2000; Davis et al. 2004; Horning et al. 2005). In patients with recurrent NHL treated with RIT, human anti-mouse antibodies (HAMA) are found in 4–11 %. Acute side effects due to HAMA (such as serum sickness-like illness) are not typically seen in this population due to a history of prior therapies and relative immune compromise, but could affect future treatment decisions (treatment with additional monoclonal antibody, eligibility for clinical trials) or interfere with certain laboratory evaluations. HAMA is far more common if ¹³¹I-tositumomab is used as the initial therapy of NHL.

Hypothyroidism has been reported to occur in ∼18 % of patients who receive ¹³¹I-tositumomab despite thyroid blockade prior to therapy, but is probably less with careful attention to thyroid blockade. Secondary myelodysplastic syndrome/acute myelogenous leukemia (MDS/AML) has been reported in 2–4 % of patients with recurrent NHL who received RIT (Zevalin package insert 2009; Bexxar package insert 2003). The patients in these studies were heavily pretreated, and the role of RIT versus exposure to prior cytotoxic agents and a history of NHL in the development of MDS/AML are unresolved.

Czuczman et al. (2007) retrospectively reviewed the data from 746 patients previously treated with ⁹⁰Y-ibritumomab tiuxetan from 1996 to 2002 who were participating in clinical trials and compassionate use trials. Nineteen patients (2.5 %) developed MDS/AML at a median of 5.6 years (1.4–13.9 years) after RIT. The annualized incidences of MDS/AML after NHL diagnosis (0.3 % per year) and after treatment (0.7 % per year) were not more than those expected in heavily pretreated patients with NHL. This experience may be relevant to the experience with ¹³¹I-tositumomab. At a median follow-up of 5.1 years, no cases of MDS/AML were reported in 76 patients with untreated follicular NHL treated with ¹³¹I-tositumomab as front-line therapy, though a single case has been reported after longer term follow-up (Kaminski et al. 2005). Patients with NHL are generally at higher risk for the development of MDS/AML and should be monitored carefully.

RIT has also been investigated in front-line and consolidation settings. In both instances, single doses of RIT are effective and can produce long-term remissions. In 76 untreated patients with follicular NHL, Kaminski et al. reported a 95 % overall response rate and a 75 % complete response rate to a single dose of ¹³¹I-tositumomab (Kaminski et al. 2005). The estimated median progression-free survival rate at 5 years was 59 % and the mean progression-free survival was 6.1 years. The relapse rate decreased progressively with time from the first RIT and suggests that the quality of initial response is important for subsequent relapse.