Hybrid tracers containing both fluorescent
and radioactive imaging
labels have demonstrated clinical potential during sentinel lymph
node procedures. To combine these two labels on a single targeting
vector that allows tumor-targeted imaging, end-labeling strategies
are often applied. For α
v
β
3
-integrin-targeting hybrid tracers, providing an excellent model
for evaluating tracer development strategies, end-labeling-based synthesis
provides a rather cumbersome synthesis strategy. Hence, the aim of
this study was to investigate the use of heterobifunctional cyanine
dyes in a click-chemistry-based synthesis strategy for RGD-based hybrid
tracers. The triazole-based hybrid tracers
DTPA.DBCO.N
3
(SO
3
)-Cy5-c[RGDyK]
and
DTPA.BCN.N
3
(SO
3
)-Cy5-c[RGDyK]
were obtained in fewer steps than
DTPA-Lys(Cy5(SO
3
)methyl)-Cys-c[RGDyK]
and had partition
coefficients of log
P
(o/w)
= −2.55
± 0.10, −1.45 ± 0.03, and −2.67 ± 0.12,
respectively. Both tracers were chemically stable, and the brightnesses
of
DTPA.DBCO.N
3
(SO
3
)-Cy5-c[RGDyK]
and
DTPA.BCN.N
3
(SO
3
)-Cy5-c[RGDyK]
were, respectively, 23 × 10
3
and 40 × 10
3
M
–1
cm
–1
; lower than that of the reference tracer
DTPA-Lys(Cy5(SO
3
)methyl)-Cys-c[RGDyK]
(50 × 10
3
M
–1
cm
–1
). Assessment of serum protein binding revealed no statistically significant
difference (44 ± 2 and 40 ± 2% bound for
DTPA.DBCO.N
3
(SO
3
)-Cy5-c[RGDyK]
and
DTPA.BCN.N
3
(SO
3
)-Cy5-c[RGDyK]
, respectively; 36 ± 5% bound for
DTPA-Lys(Cy5(SO
3
)methyl)-Cys-c[RGDyK]
;
p
> 0.05).
DTPA.DBCO.N
3
(SO
3
)-Cy5-c[RGDyK]
(
K
D
= 17.5 ± 6.0) had a statistically significantly
higher affinity than the reference compound
DTPA-Lys(Cy5(SO
3
)methyl)-Cys-c[RGDyK]
(
K
D
= 30.3 ± 5.7;
p
<
0.0001), but
DTPA.BCN.N
3
(SO
3
)-Cy5-c[RGDyK]
had
a statistically significantly lower affinity (
K...