Toward the Optimization of (+)-[11C]PHNO Synthesis: Time Reduction and Process Validation

Abstract

(+)-[¹¹C]PHNO, a dopamine D_2/3 receptor agonistic radiotracer, is applied for investigating the dopaminergic system via positron emission tomography (PET). An improved understanding of neuropsychiatric disorders associated with dysfunctions in the dopamine system and the underlying mechanism is a necessity in order to promote the development of new potential therapeutic drugs. In contrast to other broadly applied ¹¹C-radiopharmaceuticals, the production of this radiotracer requires a challenging four-step radiosynthesis involving harsh reaction conditions and reactants as well as an inert atmosphere. Consequently, the production is prone to errors and troubleshooting after failed radiosyntheses remains time consuming. Hence, we aimed to optimize the radiosynthesis of (+)-[¹¹C]PHNO for achieving better activity yields without loss of product quality. Therefore, we synthesized (+)-[¹¹C]PHNO and omitted all heating and cooling steps leading to higher activity yields. As a result, radiosynthesis fully conducted at room temperature led to a time-reduced production procedure that saves about 5 min, which is an appreciable decay-prevention of around 15% of the activity yield. Additionally, we established a troubleshooting protocol by investigating reaction intermediates, byproducts, and impurities. Indeed, partial runs enabled the assignment of byproducts to their associated error source. Finally, we were able to generate a decision tree facilitating error detection in (+)-[¹¹C]PHNO radiosynthesis.

Figure 1

Synthesis scheme for the four-step radiosynthesis of (+)-[¹¹C]PHNO starting from cyclotron-produced [¹¹C]CO₂ and ethylmagnesium bromide.

Figure 2

Chemical structure of the desired intermediate amide (compound 3) as well as nonreduced form of the side products (compounds 4 and 5) and reduced side products (compounds 6 and 7).

Figure 3

Reaction scheme of partial runs A–E for investigation of reaction intermediates formed during (+)-[¹¹C]PHNO radiosynthesis.

Figure 4

Reaction scheme of partial runs G–J for investigation of reaction intermediates formed during (+)-[¹¹C]PHNO radiosynthesis.

Figure 5

Reaction steps of (+)-[¹¹C]PHNO production according to Rami-Mark et al. [21] with the corresponding main risk for each reaction step.

Figure 6

(a) Comparison of the synthesis duration of (+)-[¹¹C]PHNO synthesis at room temperature (n = 16) and according to our previously published method including heating and cooling (n = 9). (b) Influence of the temperature at LAH addition on the isolated radiochemical yield not corrected for decay (22°C: n = 16, −15°C: n = 9, −40°C: n = 3).

Figure 7

Analytical HPLC chromatograms of the crude reaction mixtures obtained by addition of LAH at room temperature (a) or at −40°C (b).

Figure 8

Chromatogram of semipreparative HPLC of full synthesis at room temperature (a), route A synthesis without TEA and LAH (b), route E without LAH (c), and route F in which the reaction is quenched after Grignard reaction (d).

Figure 9

Semipreparative HPLC chromatogram of partial run G and the corresponding analytical chromatograms of the respective peak.

Figure 10

(a) UV/Vis chromatogram of [^natC]propionic acid. (b) Radio-channel chromatogram of [¹¹C]CO₂.

Figure 11

Schematic decision tree aiding troubleshooting after a failed (+)-[¹¹C]PHNO formation.