bc0c00325_si_001.pdf (1.88 MB)
Ultrasonic-Assisted Solid-Phase Peptide Synthesis of DOTA-TATE and DOTA-linker-TATE Derivatives as a Simple and Low-Cost Method for the Facile Synthesis of Chelator–Peptide Conjugates
journal contribution
posted on 2020-07-23, 13:09 authored by Shvan
J. Raheem, Benjamin W. Schmidt, Viswas Raja Solomon, Akam K. Salih, Eric W. PricePeptides
have been widely adopted as biological targeting vectors
for applications in molecular imaging and peptide-receptor radionuclide
therapy (PRRT). Somatostatin (SST) analogues such as octreotate (TATE)
are exogenous ligands for somatostatin receptors (SSTRs), which are
highly expressed on neuroendocrine tumors (NETs). Recently, both [68Ga]Ga-DOTA-TATE (NETSPOT) and [177Lu]Lu-DOTA-TATE
(LUTATHERA) received U.S. Food and Drug Administration approval for
positron emission tomography (PET) imaging and PRRT of NETs, respectively.
However, to the best of our knowledge a well-described synthesis of
DOTA-TATE has not been reported in the literature. Herein, we report
a fully reoptimized DOTA-TATE synthesis, including the application
of a simple ultrasonic bath to greatly improve yields, reduce coupling
times, and decrease the amount of reagents required for each coupling
step by a half. The most prevalently used cyclizing agents such as
iodine, thallium(III) trifluoroacetate, hydrogen peroxide, and dimethyl
sulfoxide were compared. On-resin cyclizations using mechanical agitation
showed higher yields (23% and 25% using I2 and Tl(III),
respectively) than off-resin (1.3% and 11% using DMSO and H2O2, respectively), and the total synthesis time of DOTA-TATE
was ∼540 min excluding the cyclization step, with a total synthesis
yield of ∼23%. The same manual SPPS methods/reagents were reoptimized
with ultrasonic (US) agitation, resulting in an immense reduction
in the total synthesis time by ∼8-fold to ∼70 min for
DOTA-TATE with a higher yield (∼29% yield), and ∼13-fold
to 105 min for DOTA-PEG4-TATE (∼29% yield). Also,
the use of US agitation reduces the need for excess molar equivalents
of the reagents to a half, which is particularly important when coupling
expensive or custom-synthesized groups such as bifunctional chelators
and linkers. Finally, the synthesized DOTA-TATE was successfully radiolabeled
with [68Ga]Ga3+ (t1/2 = 68 min) with high radiochemical yields (30 min, 95 °C). We
believe this work opens the door to the facile and low-cost synthesis
of many new chelator-linker-peptide conjugates that
were previously cumbersome or cost-prohibitive to produce with manual
SPPS.