A method for preparing chelate-cytokine conjugates with retention of protein structure, biological activity, and pharmacokinetic properties.

The chelating agent diethylenetriaminepentaacetic acid (DTPA) has been conjugated site-specifically to the N-terminus of recombinant human interleukin-2 (rhIL-2) by reaction with DTPA dianhydride at an initial pH of 6.0, thus demonstrating broader application of the conjugation method previously described for the structurally related cytokine rhG-CSF (Ralph et al., 1995). Purity of the DTPA-rhIL-2 conjugate, isolated by cation-exchange FPLC, and chelation of 111In were revealed by cation-exchange HPLC. Purity of the conjugate as well as chelation of radiometal were also demonstrated by SDS-PAGE and TLC, respectively. The stoichiometric molar ratio of DTPA to protein for the conjugate was approximately 1:1 as determined by TLC and mass spectrometry. Localization of the DTPA moiety was resolved by a peptide mapping procedure. The protein retained > 95% secondary structure (alpha helicity) following the conjugation. Addition of metal induced an approximate 22% loss of secondary structure for the conjugate. The in vitro biological activity of the protein was unaffected by the conjugated DTPA, even with chelated metal. Pharmacokinetic analysis of DTPA-conjugated cytokines, following chelated 111In, showed clearance and pharmacokinetic parameter values comparable to those of the corresponding unmodified cytokine. DTPA-conjugated cytokines may prove useful in cytokine research, and furthermore may represent a novel class of molecules for imaging, diagnosing, and/or treatment of malignancies where the cytokine receptor is overexpressed.

Site-specific conjugation of diethylenetriaminepentaacetic acid to recombinant human granulocyte-colony-stimulating factor: preservation of protein structure and function.

The chelating agent diethylenetriaminepentaacetic acid (DTPA) was conjugated site-specifically to the N-terminus of recombinant human granulocyte-colony-stimulating factor (rhG-CSF) by reaction of the protein with DTPA dianhydride at an initial pH of 6.0. The reaction was efficient in that 84% of the starting rhG-CSF was N-terminally modified and could be purified to homogeneity by cation-exchange chromatography. Chelation of 111In by the DTPA-rhG-CSF conjugate was demonstrated by cation-exchange HPLC and thin-layer chromatography. Metal contamination of conjugate preparations, as well as metal-loading onto the conjugate, could be monitored by either cation-exchange HPLC or isoelectric focusing. The 1:1 stoichiometric molar ratio of DTPA to protein for the DTPA-rhG-CSF conjugate was determined by thin-layer chromatography and mass spectrometry, and the localization of the conjugated DTPA moiety was resolved using a peptide mapping procedure. The secondary structure (i.e., alpha-helicity) of the protein was unmodified following conjugation as revealed by circular dichroism. Furthermore, the conjugate induced a similar induction of peripheral WBC counts as unmodified rhG-CSF when injected subcutaneously into hamsters, demonstrating preservation of protein bioactivity. These results reveal a simple and efficient method for conjugating DTPA to protein, via reaction with the dianhydride, to yield a homogeneous and well-defined product. The procedure may prove to be a useful method of labeling growth factors and related proteins while preserving structural and functional integrity.