Identification of radiopharmaceuticals by their retention on HPLC: A caveat

hr. J. Appl. Rndiar. hr. C Pergamon 0020-708X/84

Vol. 35. No. 6. p. 545. 1981 Press Ltd 1984. Printed in Great Bnrain 93.00 + 0.00

Identification of Radiopharmaceuticals by their Retention on HPLC: A Caveat GERD KLOSTER and PETER LAUFER Institut fur Chemie 1. Nuklearchemie der Kemforschungsanlage D-5170 Jiilich, F.R.G. (Receired

I2 September

Jiilich.

1983)

It is common practice in laboratories preparing radiopharmaceuticals labelled with short-lived cyclotronproduced radionuclides at the no-carrier-added (nca) level to identify these labelled compounds on HPLC in the absence of a U.V. absorption by the identity of retention volumes or retention times of the radioactive peak with that

of standard compounds.“’ Standard compounds. however. are usually injected in amounts orders of magnitude larger than those expected for nca radiopharmaceuucals in order to allow easy detection. It has been tacitly assumed that the retention times or volumes (or better capacity factors k’) are independent of the mass of the injected compounds unless one overloads the column. During recent efforts to investigate pancreas imaging and L-[3-‘“I]iod~~-methylt~Tosine’?’ agents, namely L-[3-‘5Br]bromo-x-methyltyrosine (BMT),“” we observed a case where this assumption proved to be wrong. While preparing BMT from nca [“Brlbromide, L-z-methyltyrosine (MT) and chloramine-T (detailed procedure: see Ref. 4). we used’ a Lichrosorb RP 18, 10 p column (25 x 1.0 cm) ‘with water: methanol: acetic acid 80: 20: I (v/v/v) as eluent for HPLC purification of BMT. Using a cold BMT standard (about 0.5 mg total mass) we had observed a k’ of 2. The radioactive peak of the nca [‘5Br]BMT, however, was observed at a k’ of 6.5. Reinjection of the isolated radioactive material with 0.5 me of cold BiMT led to the observation of a coinciding U.V. aid radioactivity peak at k’ = 2. Prompted by this intriguing observation. we investigated the chromatographic behaviour of BMT and the starting material MT as a function of the total mass injected. Results are shown in Fig. I. With MT, k’-values changed from 2.2 at 0.6~9 to 0.8 at I.1 mg. For BMT, the change was more pronounced from 6.5 at the nca level to 1.6 at 1-2 mg. The region of rapid change is in the range of 5Opg-I mg. Reasons for this chromatographic behaviour are unknown at present. Although this is the only example of dependence of chromatographic retention (k’) on the mass injected we have observed so far, we feel that this exception to the rule should make others in the field cautious when using this common practice of identification of radiopharmaceuticals by HLPC. References

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I. Meyer G. J. Rudiochim. Acra 30, 175 (1982). 2. Tisliar U.. Kloster G.. Ritzl F. and Stiicklin G. J. Nucl.

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Me>. to, 973 (1979). 3. Kloster G., Coenen H. H., Szabo, Z., Ritz1 F. and Stiicklin G. In Progress in Radiopharmacology (Ed. Cox P. H.) Vol. 3. p. 97 (Nijhoff, Den Haag. 1982). 4. Petzold G. and Coenen H. H. J. Labelled Compd.

( mass, g )

Fig. I. Chromatographic retention (k’) of BMT (0) and MT (m) as a function of injected mass. Column: LiChroSorb RP 18, 10 p, 25 x I cm; eluent :water: methanol: acetic acid 80:20: I (v/v/v).

Radiophurm. 18, 1319 (1981).

545