Intra-individual comparison of the human biodistribution and dosimetry of the D and L isomers of 2-[123I]iodo-phenylalanine

Purpose Several authors have shown in animal studies that D-enantiomeric amino acid analogues can have better tumour imaging properties compared to their L-analogues. In our group, the D and L isomers of 2-[I-123]iodophenylalanine were identified as tumour-specific tracers in rat and mouse tumour models, with an advantage for the D-isomer. Here we compare, intra-individually, the normal biodistribution and dosimetry of both tracers in healthy human subjects. Methods Five male volunteers received both the L- and D-enantiomers, ranging from 84 to 114 MBq, with a 1 week interval between the tracers, allowing intra-individual comparison. Whole-body scans were performed and blood and urine samples were collected and analysed up to 24 h. Dosimetry was calculated using OLINDA 1.0 software. Results The biodistributions of the tracers are comparable as both show a moderate uptake in heart and the liver, a marked uptake in muscle tissue and clearance via the renal system. However, due to faster clearance, from 2.5 h, the uptake of the D-enantiomer was significantly lower compared to the L-isomer in all organs. The radiation dose estimations showed an effective dose of, respectively, 0.0120 +/- 0.0020 mSv.Bq(-1) and 0.0106 +/- 0.0038 mSv.Bq(-1) for 2-I-123-L-Phe and 2-I-123-D-Phe (P=0.18). In both cases the organ receiving the highest dose was the bladder wall. Conclusion Both 2-I-123-L-Phe and 2-I-123-D-Phe show comparable moderate uptake in all organs. 2-I-123 -D-Phe is the more promising tracer, as it shows a faster clearance resulting in a lower dose and a lower background, favouring tumour imaging with respect to the tumour/ background ratio.