Halogens are usually provided under the halogenide form. The easiest way to attach a halogen to a vector is to perform an electrophilic substitution on a tyrosine residue [23]. Unfortunately, even if this commonly used technology is validated with iodine-labeled non-internalizing antibodies or peptides, it does not provide satisfactory results when internalization occurs or with ²¹¹At-labeled antibodies [24]. In both cases, halogen liberation leads to nonspecific irradiation of normal organs such as the thyroid for iodine [25] or the stomach for astatine [26] and reduces specific irradiation of the tumor. Several radiolabeling approaches using prosthetic groups have been proposed to solve this problem (Fig. 25.1).

Radiometals, such as ⁹⁰Y, ¹⁸⁸Re, ⁶⁷Cu, ¹⁷⁷Lu, ²¹²Bi, and other actinides, are generally provided no carrier added in chloride form. However, contaminations with metal traces resulting from the production mode decrease the specific activity of radiopharmaceuticals, which are usually between 40 and 400 MBq/nmol depending on the radionuclide. Several highly specific chelating agents have been developed in order to improve specific activity [27].

Transmetallation or transchelation phenomena can occur in vivo when the radiopharmaceutical is in competition with metal complexing proteins, such as transferrin or ceruloplasmin [28, 29]. Thus, chelating agents with very high affinities for metals and very high kinetic stabilities have been developed (Table 25.1). The best approaches to limit these phenomena are based on a better chelation agent selection in order to improve both selectivity and stability. This choice integrates stability constant and dissociation kinetic values which have to be for the latter as low as possible.

Bexxar® and Zevalin® are administered 6–8 days after a predose of cold mAbs, respectively 2 × 450 mg of tositumomab and 2 × 250 mg of rituximab, to improve biodistribution and tumor targeting. Bexxar® and Zevalin® can be integrated in clinical practice using non-ablative doses for treatment of patients with relapsed or refractory FL or as consolidation after induction chemotherapy in frontline treatment in FL patients. Hematological toxicity is the major side effect of RIT and depends on bone marrow involvement and prior treatment [30–32]. Non-hematological toxicity is generally low. Secondary myelodysplastic syndrome or acute myelogenous leukemia (AML) was reported in 1–3 % of cases [30–33]. The risk appears to be increased in patients previously treated by several lines of chemotherapy or radiotherapy. In a meta-analysis involving relapsed B cell lymphoma patients treated with Zevalin® in four clinical trials, long-term responses (time to progression (TTP) >12 months) were seen in 37 % of patients [32]. At a median follow-up time of 53.5 months, the median TTP was 29.3 months. One third of these patients had been treated with at least three previous therapies, and 37 % of them had not responded to their last therapy. The estimated 5-year overall survival (OS) was 53 % for all patients treated with Zevalin® and 81 % for long-term responders. Using Bexxar® in a long-term meta-analysis performed on 250 heavily pretreated patients with indolent lymphoma in 5 clinical trials, objective response (OR) rates ranged from 47 to 68 % and complete response (CR) rates from 20 to 38 % [34]. Interestingly, poor prognostic patients showed durable responses (bone marrow involvement in 41 %, bulky disease ≥5 cm in 49 %, and transformed histology in 23 %).

Clinical results showed that Zevalin® or Bexxar® had a significant efficacy but moderate response duration as a monotherapy in rituximab-refractory recurrence of FL. A higher therapeutic impact may be achieved using Bexxar® or Zevalin® in other indications. Recent studies showed that RIT can be administrated as high-dose treatment. This approach consists of injecting myeloablative activity of RIT or combining standard or escalated activity of RIT with high-dose chemotherapy. In a recent prospective multicenter study, Shimoni et al. demonstrated that standard-dose Zevalin® (0.4 mCi/kg) combined with BEAM high-dose chemotherapy was safe and possibly more effective than BEAM alone as a conditioning regimen for stem cell transplantation (SCT) in 43 patients with relapsed/refractory aggressive non-Hodgkin lymphoma [35]. The 2-year progression-free survival (PFS) was 59 and 37 % in the Z- BEAM and BEAM arms, and the 2-year OS was 91 and 62 %, respectively.

RIT can also be administered as consolidation after induction therapy. The FIT randomized phase III trial showed the benefits of Zevalin® as consolidation in previously untreated FL patients [7]. A high conversion rate from partial response (PR) to CR of 77 % was observed after RIT, leading to a high CR rate of 87 %. Moreover, different studies suggest that RIT is a relevant option as consolidation therapy in different subtypes of B cell lymphoma such as diffuse large B cell or mantle cell lymphoma, in order to decrease the number of chemotherapy courses in elderly patients or as an alternative of stem cell transplantation in high-risk patients [36, 37]. In 2010, Zinzani et al. published the results of a phase II study assessing the efficacy and safety of Zevalin® following four cycles of R-CHOP21, in 55 high-risk elderly (age ≥60 years) patients with previously untreated diffuse large B cell lymphoma (DLBCL). Forty-eight of the 55 patients received RIT [38]. The OR rate for the entire treatment regimen was 80 %, including 73 % CR. Eight of the 16 patients (50 %) who achieved less than a CR after R-CHOP improved their remission status after RIT. With a median follow-up of 18 months, the 2-year PFS was estimated to be 85 %, with a 2-year OS of 86 %.

RIT can also be considered alone in frontline treatment. Recently, Scholz et al. evaluated, in an international multicenter phase II clinical trial, the efficacy and feasibility of Zevalin® as first-line treatment in 59 FL patients [39]. Treatment indication resulted from B symptoms, grade 3A, organ compression or infiltration, rapid growth, and/or bulky disease. The OR rate at 6 months after RIT was 87 %, with 41 % of the patients achieving CR, 15 % unconfirmed CR, and 31 % PR. Median PFS was 25.9 months. RIT was well tolerated and the most common toxicity was hematological and reversible.

PCa accounts for an estimated 70,347 deaths in Europe in 2013 [67]. Up to 40 % of patients eventually develop metastases despite local therapy. Once metastases have developed, PCa is incurable and all therapy is palliative. Medical castration is highly effective in shrinking tumor burden, decreasing prostate-specific antigen (PSA) levels, enhancing quality of life, and improving survival [68]. However, most patients evolve toward progression despite castration, with a median duration of response of 12–24 months [68]. At the stage of castration-resistant PCa (CRPC), cytotoxic chemotherapy was the only therapy [69, 70] until 2012, when the European Medicines Agency (EMA) approved the use at this stage of abiraterone acetate before docetaxel. Within the past year, three new drugs were FDA approved for the treatment of patients with CRPC (cabazitaxel, sipuleucel-T, and denosumab). However, the survival benefit of these drugs in CRPC is modest: respectively +2.4, +4.1, and +3.6 months, and more efficacious drugs are needed.

Radiotherapy is an established treatment for clinically localized PCa or for palliation of painful bone metastasis [71]. PCa is a favorable solid malignancy for which RIT may be used because it is a radiosensitive tumor with typical distribution to sites with high exposure to circulating radiolabeled mAbs (bone marrow and lymph nodes). In preclinical and clinical PCa therapy studies, radionuclides have been linked to antibodies or peptides with affinity to mucin, ganglioside (L6), Lewis Y (Ley), adenocarcinoma-associated antigens, and prostate-specific membrane antigen (PSMA) [72–75], but PSMA appears the most specific.

PSMA is an integral, non-secreted, type II membrane protein with abundant and nearly universal expression on prostate epithelial cells and is strongly upregulated in PCa [76–80]. Pathology studies indicate that PSMA is expressed by virtually all PCa [81]. The level of expression in non-prostate tissues is 100–1,000-fold less than in prostate tissue [76], and the site of PSMA expression in normal cells (brush border/luminal location) is not typically exposed to circulating mAb. De-immunized J591 mAb, which targets the external domain of PSMA, giving an easy and rapid access to the antigen, seems to be the best clinical candidate for imaging and therapy of PCa [82, 83].

A phase I trial assessing ¹¹¹In/⁹⁰Y-J591 was performed in 29 patients [84]. Dose-limiting toxicity was seen at 740 MBq/m², and 647.5 MBq/m² was determined as the maximal tolerated dose (MTD). The overall targeting sensitivity of bone and soft tissue metastasis was 81 %. Decrease of PSA was observed for two patients as objective measurable disease responses with decrease of lymph node size.

Thirty-five patients were enrolled in a ¹⁷⁷Lu-J591 phase I trial [85]. The 2,590 MBq/m² level was determined as MTD. Repeated dosing up to three doses of 1,110 MBq/m² could be safely administered. Clearly identified sites of metastatic disease were successfully imaged by ¹⁷⁷Lu-J591 scintigraphy in 100 % of patients. The median duration of PSA stabilization, after treatment, was 60 days with a range of 28–601 days. No immune response was detected. A phase II ¹⁷⁷Lu-J591 trial was initiated in CRPC patients (ASCO congress 2008). Fifteen patients (cohort 1) were treated with 2,405 MBq/m². The second cohort (2,590 MBq/m²) enrolled 17 patients (ASCO congress 2013). Sensitivity of known metastasis targeting was 93.6 %. Reversible thrombocytopenia and neutropenia toxicity occurred respectively in 46.8 and 25.5 %. The second cohort dose (2,590 MBq/m²) showed more PSA responses (46.9 % vs. 13.3 %, p = 0.048) associated with a longer survival (21.8 vs. 11.9 months, p = 0.03) but also more reversible hematological toxicity.

These trials provide support that radiolabeled de-immunized J591 is well tolerated and non-immunogenic. Radiolabeled J591 effectively targets PCa metastases with high sensitivity and specificity, produces PSA, and declines with a dose-effect relationship.

Alpha-RIT is a therapeutic modality based on the use of an antitumor antigen mAbs coupled to an alpha emitter (radionuclide which decays by the emission of alpha particles). The rationale of this therapeutic modality is based on two prominent characteristics of the alpha particles: their short range in tissue, inferior to 100 μm, which allows for a good specificity of the treatment (once the antibody is in the vicinity of the tumor) and their high linear energy transfer (LET) between 50 and 250 keV/μm, which makes them highly cytotoxic. In vitro studies have demonstrated that 1–20 cell nucleus traversals by alpha particles are sufficient to inactivate a cell as compared to thousands or tens of thousands for the same effect with beta⁻ particles [86, 87]. In addition, alpha particle-induced toxicity was shown to be independent of both dose rate and oxygenation of the irradiated tissue [88].

Medullary thyroid carcinoma (MTC) represents less than 10 % of all thyroid carcinoma. Prognosis of metastatic disease varies from long- to short-term survival. Among the various prognostic parameters, advanced age, stage of the disease, EORTC prognostic scoring system mutations in the RET oncogene, and association with multiple endocrine neoplasia (MEN) 2B are commonly accepted as prognostic factors [104–108]. Moreover, Barbet et al. demonstrated that calcitonin (Ct) serum level doubling times (DT) was an independent predictor of OS [109]. In this study, all the 41 patients with Ct DT >2 years were still alive at the end of the study 2.9–29.5 years after initial surgery. Eight patients (67 %) with DT between 6 months and 2 years died of the disease 40–189 months after surgery, and all 12 patients with Ct DT <6 months died of the disease 6 months to 13.3 years after initial surgery. Giraudet et al. confirmed the prognostic value of biomarker DT in metastatic MTC [110].

Targeted therapy using multikinase inhibitors can be applied in progressive patients and vandetanib has been approved [111–116]. MTC cells express high amounts of CEA, and anti-CEA radiolabeled mAbs have shown promising results [117, 118]. Pretargeted RIT (pRIT) was developed to improve the tumor-to-normal-tissue ratios and to deliver increased tumor-absorbed doses to relatively radioresistant solid tumors. Pretargeted system involves a first injection of an unlabeled bispecific monoclonal antibody (BsmAb), followed by a second injection of a radiolabeled bivalent hapten peptide [1, 19, 119–121]. Using this system, the radiolabeled bivalent peptide binds avidly to the BsmAb attached to the CEA antigen on the cell surface, whereas non-targeted hapten-peptide in the circulation clears rapidly through the kidneys.

A phase I/II clinical trial was started in 1996 to evaluate pRIT using the murine anti-CEA x anti-indium-DTPA F6 × 734 BsmAb and a bivalent indium-DTPA hapten labeled with iodine-131, in 26 metastatic MTC patients [122–124]. A good tumor targeting was observed. Dose-limiting toxicity was hematological, and maximum tolerated activity was estimated at 1.8 GBq/m² in this population of patients with high frequency of bone marrow involvement. Some tumor responses were observed, mainly in patients with a small tumor burden and after repeated courses of pRIT. Because of a relatively high hematological toxicity and frequent immune responses, the chimeric hMN-14 × m734 BsmAb was developed and assessed in a prospective phase I study performed in 34 patients with CEA-expressing tumors to determine optimal BsmAb dose, hapten activity, and pretargeting interval [22]. A BsmAb dose of 40 mg/m² with a pretargeting interval of 5 days appeared to be a good compromise between toxicity and efficacy. HAMA elevation was observed in 8 % of patients and HAHA (human antihuman antibody) in 33 %.

In 2006, OS of the series of 29 MTC patients involved in the two phase I/II pRIT trials was retrospectively compared with that of 39 contemporaneous untreated patients (data collected by the French Endocrine Tumor Group, GTE) [125]. A second objective was to examine whether post-pRIT Ct DT variation was a surrogate marker for survival. Patients with Ct DT <2 years were considered as high-risk patients. This study showed that OS was significantly longer in high-risk treated patients than in high-risk untreated patients (median OS, 110 vs. 61 months; P < 0.030).

Following theses encouraging results, a prospective phase II multicenter pRIT trial was designed in progressive MTC patients with Ct DT shorter than 5 years. Forty-two MTC patients received 40 mg/m² of hMN-14 × m734 and 1.8 GBq/m^{2 131}I-di- indium-DTPA hapten 4–6 days later [126]. Disease control according RECIST criteria (objective response + stabilization) was observed in 32 patients (76.2 %), including a durable CR of at least 40 months in 1 patient (2.4 %) and durable stable disease (≥6 months) in 31 patients (73.8 %). Tumor uptake assessed by post-pRIT immunoscintigraphy was a significant predictor of response. As previously reported, toxicity was mainly hematological, requiring careful post-RIT blood monitoring. Pre-RIT biomarker DT and impact on DT after pRIT were predictors of OS, confirming the value of serum biomarkers in selecting patients and monitoring therapy.

Today, a new generation of compounds is available for pRIT. Humanized, recombinant, trivalent BsmAb (anti-CEA TF2) and bivalent histamine-succinyl-glutamine (HSG) peptides have been produced [127, 128]. The use of TF2, composed of a humanized anti-HSG Fab fragment derived from the 679 anti-HSG mAb and two humanized anti-CEA Fab fragments derived from the hMN-14 mAb (labetuzumab, Immunomedics, Inc.) by the dock-and-lock procedure, should reduce immunogenicity [127–129]. Moreover, the HSG peptide allows facile and stable labeling with different radiometals, such as ¹⁷⁷Lu or ⁹⁰Y, having favorable physical features that could improve pRIT efficacy [130].