A new and convenient method for purification of 86Y using a Sr(II) selective resin and comparison of biodistribution of 86Y and 111In labeled Herceptin™

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Abstract

A simple and rapid procedure was developed for purification of cyclotron produced 86Y via the 86Sr(p,n) 86Y reaction. A commercially available Sr(II) selective resin was used to separate 86Y from the cyclotron irradiated Sr(II) target with a recovery of the enriched Sr(II) target while yielding a 75–80% recovery of 86Y suitable for radiolabeling either proteins or peptides. To demonstrate the utility of this methodology, the anti-HER2 monoclonal antibody Herceptin™ was radiolabeled with the purified 86Y and compared to 111In labeled Herceptin™. The biodistribution study demonstrated that 111In-Herceptin™, while a suitable surrogate for 90Y in the major organs, did not parallel the uptake of 86Y-Herceptin™in the bone, and thus may not accurately predict the level of 90Y accumulation in the bone for clinical RIT applications. This result exemplifies the requirement of employing appropriate matched pair isotopes for imaging and therapy to insure that dosimetry considerations may be addressed accurately.

Introduction

The use of radiolabeled monoclonal antibodies (mAb’s) for radioimmunotherapy (RIT) continues to be a burgeoning area of research activity [9]. This is particularly evident in the development of commercial agents such as Zevalin™, an anti-CD20 monoclonal antibody that is radiolabeled with the pure β-emitter 90Y (t1/2 = 64.1 hr, Emax = 2.27 MeV) as a therapeutic for treating non-Hodgkins lymphoma [6]. These emissions are quite limited with a maximum range on the order of a centimeter with a mean range of less than half that distance. However, as expected, the direct imaging of these emissions has been quite unsatisfactory. Several attempts to employ the Bremsstrahlung radiation associated with 90Y have been reported and this option has generally been relegated to be either impractical or of limited value [21], [22]. In order to obviate these issues and to acquire reasonably reliable information useful for both the diagnostic and dosimetric considerations associated with RIT applications, researchers routinely use 111In (t1/2 = 2.83 d, γ-rays 171, 245 keV) for obtaining scintigraphic images. Fortunately, the coordination chemistries of In(III) and Y(III) are adequately similar to generally permit use of their respective isotopes within the same chelating agents conjugated to mAb’s [14]. For these reasons, 111In has routinely been employed as the imaging partner to the therapeutic use of 90Y [2], [14], [15], [25].

However, there is also reasonable concern about the relative stabilities of the complexes of these two isotopes in the same chelating agent when conjugated to a mAb. While the coordination chemistries may be similar, the requirement for a higher degree of stability in complexing the lanthanide isotopes as opposed to 111In has been well demonstrated [20] as well as the fact that the biological accumulation patterns for these two elements are dissimilar [4], [8]. Of particular concern is the recognized property of free 90Y in vivo to be deposited in the bone. Loss of this radionuclide from the chelating agent during RIT can result in increased doses to the bone and due to the range of emission, to the bone marrow. Small differences in the in vivo complex stability of 111In and 90Y may then lead to larger than expected doses to dose-limiting tissues since the imaging phase with 111In could potentially underestimate the amount of 90Y actually at the bone.

Ideally, one would prefer to employ an imageable isotope of the same element being used as a therapeutic. The use of the positron emitting radioisotope 86Y (t1/2 = 14.7 h, β+ = 33%, Eβ+ = 1.2 MeV) has been proposed for this purpose [7], [20]. Additionally, 86Y can be produced by a cyclotron thus making this isotope potentially readily available to many institutions. However, routine production of 86Y suitable for radiolabeling mAb’s has remained less than optimal. Principally this has been because the established literature methods use co-precipitation purification methods with iron to separate the radio-yttrium from the target [5], [19]. Two of the most critical aspects to successful use of radio-metals in any targeted radiotherapy or imaging application will always be the radio-chemical purity of the isotope and its chemical purity. The presence of extraneous metal ions is a cause of poor radiolabeling efficiencies and, if severe, can completely inhibit the radiolabeling of chelate conjugated proteins. Thus, when considering the use of 86Y, a simple, rapid, and highly efficient purification method is of paramount importance to minimize loss of activity. Additionally, since the 86Y will be used for radiolabeling of an antibody, the radionuclide should also be carrier-free and chemically free of all other competing metal ions.

To meet these requirements, we have developed a rapid and efficient ion-exchange method for isolating cyclotron produced 86Y that eliminates introduction of other competing metal ions and also provides the 86Y in a milieu suitable for radiolabeling mAbs. Additionally, this method also allows for the recovery of the enriched target material thereby permitting recycling of a costly resource. Finally, to assess whether the use of 86Y might provide a more accurate assessment of bone uptake in a RIT application, we have also compared the biodistribution of the 86Y- and 111In-labeled mAb using the bifunctional chelating agent 2-(p-isothiocyanatobenzyl)-6-methyl-diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (1B4M-DTPA).

Herein, we report the development of a novel method for the isolation and efficient radiolabeling use of 86Y. The detailed results of the comparative biodistributions of the 86Y- and 111In-labeled mAb are also reported to provide a measurement of the relative amounts of bone deposition of these two isotopes and to indicate whether the use of PET imaging with 86Y prior to therapy with 90Y may be a more accurate dosimetric tool than imaging with 111In-labeled mAb.

Section snippets

Isotope production

86Y was produced via the 86Sr(p,n) 86Y reaction by irradiating enriched 86SrCO3 with 13.8 MeV protons [7], [20]. Enriched 86Sr carbonate (lot #: 188–052-1; 86Sr (95.4%), 87Sr (1.22%), 88Sr (3.38%)) was obtained from Isonics Corporation (Golden, CO). The target was irradiated at the NIH cyclotron facility using a CS-30 cyclotron from The Cyclotron Corporation.

Target system

A water-cooled aluminum external cup target (Fig. 1) was used to contain the target material for irradiation. This employed a single-use

Statistical analysis

The data was analyzed by application of Students t-test using Sigmaplot version 4.01 (SPSS, San Raphael, CA).

Results and discussion

The production of the 86Y was performed by irradiation of enriched Sr(II) (86Sr) using the well established 86Sr(p,n) 86Y reaction [7], [19]. The isolation of the 86Y deviated from the established or the recently reported methods by virtue of the application of an ion-exchange method. This method was devised to eliminate the multiple precipitation methods and to decrease the amount of manipulations necessary to actually deliver 86Y in a ready to use form for radiolabeling protein. The

Conclusion

Yttrium-86 can be routinely produced and purified from the enriched target 86Sr. Both the target material and the Sr(II) selective resin employed to separate the 86Y can be efficiently recovered and recycled making this methodology very cost effective. This procedure eliminates co-precipitation with Fe(III) therefore eliminating the potential for introduction of extraneous metal ion contaminants which could lower radiolabeling efficiencies. The biodistribution study demonstrates that 111

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      Over the last few decades, several methods for the separation of 86Y from Sr-based targets (mainly from enriched 86SrCO3) have been reported or reviewed and are summarized in Table 1 (Qaim and Spahn, 2017; Rösch et al., 2017). They include co-precipitation and ion exchange (Rösch et al., 1993; Kettern et al., 2002), electrolysis (Reischl et al., 2002; Yoo et al., 2005; Lukić et al., 2009), single column chromatography (Kandil et al., 2009; Medvedev et al., 2011), multiple column chromatography (Garmestani et al., 2002; Park et al., 2004; Sadeghi et al., 2009a, 2009b), solvent extraction and precipitation of the 86Y (Avila-Rodriguez et al., 2008; Kandil et al., 2009; Sadeghi et al., 2010). Typically, these methods require multiple steps, making them time-consuming and difficult to automate.

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