⁹⁰ Yttrium (⁹⁰Y) is a generator nuclide that is commercially available. It has a high cross-fire effect and is one of the most qualified β ⁻ -emitter for use in clinical trials of myeloablative conditioning so far. As a pure β ⁻ -emitter, the unspecific irradiation exposure of patients, medical staff, and family members is low which allows application in outpatient protocols. The half life (t_1/2) (Table 2) is short enough to transfer the patient from the nuclear medicine ward to the transplantation ward before the hypoplasia/aplasia becomes clinically relevant. In addition, it does not compromise the hematopoietic engraftment. ⁹⁰Y is stably conjugated to the antibody by chelators derived from DTPA (diethylenetriaminepentaacetic acid) or DOTA (1,4,7,10-tetraazacyclododecane-tetraacetic acid). The formation of DOTA complexes is a slow reaction and the use of the acyclic chelators derived from DTPA therefore is favored for the labeling of peptides. The backbone substituted derivates of the chelator DTPA provide a higher complex stability compared to DTPA and can, therefore, be used for in vivo applications. Several methyl substituted isomers of DTPA have been studied. The most common derivatives are ITC-CHX-DTPA ((2-(4-isothiocyanatobenzyl)-cyclohexyldiethylenetriaminepentaacetic acid), ITC-1B4 M-DTPA, also known as MX-DTPA (2-(4-isothiocyanatobenzyl)-6-methyl-diethylenetriamine pentaacetic acid) and ITC-2B3 M-DTPA (2-(4-isothiocyanatobenzyl)-3-methyl-diethylenetriamine pentaacetic acid). CHX-DTPA has been shown to be less suited than ITC-1B4 M-DTPA (MX-DTPA) with regard to serum stability and biodistribution (Camera et al. 1994).

Because ⁹⁰Y is a pure β ⁻ -emitter, it is not suitable for dosimetry. Exact data can be obtained from positron emission tomography with ⁸⁶yttrium (⁸⁶Y). ⁸⁶Y is chemically identical to ⁹⁰Y, but is expensive, has a restricted availability, induces a high radiation exposure for the patient and medical staff and requires complex measurement techniques (Loevinger and Berman 1976). It is not practicable in clinical routine. Therefore, ¹¹¹In is most frequently used for dosimetry prior to ⁹⁰Y-therapy. It emits γ-quants with 171 and 245 keV and can be imaged by γ -cameras.

¹³¹ Iodide (¹³¹I) is a commercially available reactor nuclide. It emits both, β ⁻– and γ-irradiation (predominantly 364 keV) (Table 2). It is efficient for dosimetry as well as for therapy. The cross-fire effect is low, and the long t_1/2 of 8 days of the γ-component requires an isolation of the patients on the nuclear medicine ward for 1–2 weeks. Free ¹³¹I accumulates in the thyroid which has to be blocked by a prophylactic, oral application of sodium perchlorate. Radiolabeling procedures are simple. Since labeling techniques for ⁹⁰Y have been established, ¹³¹I is rarely used for myeloablative conditioning.

¹⁸⁸ Rhenium (¹⁸⁸Re) is a generator nuclide with limited commercial availability (¹⁸⁸Wi/¹⁸⁸Re-generator, Oak Ridge National Laboratory, Oak Ridge, TN, USA). It emits both, β ⁻ – and γ-irradiation (155 keV) and can be used for dosimetry and therapy. Radiolabeling of the antibody is carried out by complex formation with disulfide bridges. The cross-fire effect and its half-life (Table 2) are sufficient for myeloablative schemes. Free, circulating perrhenate accumulates in the thyroid, and a thyroid blockade with sodium perchlorate is essential.

The most serious disadvantage in comparison with ⁹⁰Y-labeled antibodies is the significant higher nephrotoxicity of ¹⁸⁸Re-labeled antibodies and the restricted availability (Zenz et al. 2006).

¹⁷⁷ Lutetium (¹⁷⁷Lu) is a commercially available reactor nuclide. It is conjugated to the mAb by the same chelators as ⁹⁰Y. ¹⁷⁷Lu is a β ⁻ -emitter with a γ-component with 208 keV in 11 % of decays. It is suitable for both dosimetry and therapy. Particle energy, tissue range and the cross-fire effect are considerably lower than for ⁹⁰Y and ¹⁸⁸Re but more similar to ¹³¹I. Compared with ¹³¹I, the conjugation to the antibody is more stable and the t_½ is shorter. ¹⁷⁷Lu is suitable for the treatment of small tumor conglomerates, extramedullar minimal residual disease, and marginal medullar tumor cells. It has not been used in the conditioning of patients prior to stem cell transplantation so far, but is well established in internal radiotherapies of neuroendocrine tumors (Frilling et al. 2006).