Rapid Cu-Catalyzed [²¹¹At]Astatination and [¹²⁵I]Iodination of Boronic Esters at Room Temperature

Abstract

Access to ²¹¹At and ¹²⁵I-radiolabeled compounds in excellent RCCs and RCYs was achieved in just 10 minutes at room temperature using a Cu catalyst. Reaction conditions are applicable to a broad class of aryl and heteroaryl boronic reagents with varying steric and electronic properties, as well as late-stage astatination and iodination of anticancer PARP inhibitors. This protocol eliminates the traditional need for toxic organotin reagents, elevated temperatures, and extended reaction times, providing a more practical and environmentally friendly approach to developing alpha-emitting radiotherapeutics.

Graphical Abstract

Radiohalides are powerful imaging, diagnostic, and therapeutic tools with vast applications in nuclear medicine.¹ Of such, the α-emitting radionuclide ²¹¹At provides an attractive approach to developing cancer therapeutics by delivering high-linear energy transfer (LET) radiation that, on the atomic level, is more efficient at causing DNA damage than β⁻-emitters.² While this radiation dose is highly cytotoxic to the targeted cancer cell, the surrounding healthy tissue is spared due to the 50–70 μm path-length of the alpha-particle.³ Compared to other α-emitting nuclides such as ²¹²Bi (t_1/2 = 1 h), ²¹³Bi (t_1/2 = 46 min), and ²²⁶Th (t_1/2 = 30 min), ²¹¹At (t_1/2 = 7.2 h) displays a half-life that is economically viable for radiochemistry, quality control, and transportation, which allows the radionuclide to target less accessible tumor cells in living systems.⁴ The 7.2 h half-life of ²¹¹At is better suited with fast targeting vectors that are retained in tumors such as small molecules. Although many groups have explored astatinated peptides and antibodies, a recent study has demonstrated oxidative lysosomal degradation of these targeting agents results in dehalogenation in vivo.⁵ While ²²⁵Ac (t_1/2 = 10 d), ²²³Ra (t_1/2 = 11.4 d), and ²²⁷Th (t_1/2 = 18.7 d) illustrate more favorable half-lives for antibodies, these α-emitters require ligand chelation in order to form stable chemical complexes that often escape the respective radiobio-conjugate. These radionuclides also emit unwanted decay products and are ineffective after a single alpha-decay, resulting in toxic daughter isotopes, which can distribute throughout the body.⁴ Despite the clear advantages ²¹¹At offers, synthetic methodologies that can provide access to astatinated compounds are exceedingly rare, mainly due to At having no stable isotopes, the scarcity of radio-isotope production sites capable of making ²¹¹At, and lack of understanding the chemical behavior of the element.⁶ An efficient late-stage astatination strategy would provide a much-needed platform for developing ²¹¹At-based α-emitting therapeutics.

Traditional routes to astatinated compounds proceed through electrophilic destannylation of an organotin functional group allowing for rapid incorporation of ²¹¹At (Scheme 1a).^{1, 7, 8} However, handling of toxic alkyltin reagents is required to develop the aryl stannane precursors, and only modest yields of the ²¹¹At-radiolabeled products are achieved. This is attributed to the multiple oxidation states astatine can adopt, therefore, making it difficult to obtain the desired, and non-stable, At⁺¹ species for electrophilic substitution.^{9, 10} In 2016, Brechbiel and co-workers illustrated the use of aryl iodonium salts to access astatinated aryl compounds through an S_NAr mechanistic pathway (Scheme 1b).¹¹ This detailed report also demonstrated how regioselectivity was driven by electronic effects, as ²¹¹At-product formation proceeded with high selectivity for the aryl ring containing the most electron-withdrawing substituent in the para-position. Despite the high yields obtained using this method, astantination of the conjugate aryl group results in unwanted side-product formation.

Scheme 1.

Synthetic routes to ²¹¹At-labeled compounds

Due to previous reports illustrating the versatility of the Chan–Evans–Lam reaction^12–14 to access radio-iodinated and -fluorinated compounds via boronic reagents,^15–22 we adapted a similar strategy to access ²¹¹At-labeled hetero(aryl) synthons. This study will complement these previous studies, as well as the Cu-assisted halogenation reports illustrated by Sanford and Scott, ^{23, 24, 25} demonstrating facile access to versatile α-emitting synthetic building blocks.

In 2016, Zhang and co-workers outlined a Cu/ligand-catalyzed radioiodination methodology using boronic acid precursors, achieving good radiochemical conversions (RCCs) in one hour at room temp.²² Therefore, we began our initial investigation by screening several Cu-sources in order to identify an efficient catalyst for ¹²⁵I-radiolabling aryl boronic esters that could be translated to astatination. Using Cu(pyridine)₄OTf complex, we were able to obtain excellent RCCs of 2 (Table 1, entry 5) in just 10 minutes at room temp with no ligand additives. Analogous boryl reagents of 1, (entries 9–11), were also compatible with the outlined ¹²⁵I-radiolabeling protocol. We then investigated 4-methoxyphenylboronic N-methyliminodiacetic acid (MIDA) ester (entry 12), a pyramidalized class of boryl reagents, however, low RCC values were obtained (entry 12).

Table 1.

Optimization of ¹²⁵I-Radiolabling of 4-Methoxyphenyl Boryl Reagents^a

entry	R	Cu source	mol %	RCCb (%)
1	Bpin	none	0	NC c
2	Bpin	Cu2O	5	NC c
3	Bpin	Cu(CO2CH3)2	5	7 ± 1
4	Bpin	Cu(OCOCF3)2·H2O	5	50 ± 1
5	Bpin	Cu(CH3CN)4OTf	5	91 ± 2
6	Bpin	Cu(pyridine)4(OTf)2	5	95 ± 1
7	Bpin	Cu(pyridine)4(OTf)2	1	80 ± 8
8	noned	Cu(pyridine)4(OTf)2	5	NC c
9	B(OH)2	Cu(pyridine)4(OTf)2	5	99 ± 1
10	BF3K	Cu(pyridine)4(OTf)2	5	89 ± 4
11	Bnepe	Cu(pyridine)4(OTf)2	5	99 ± 1
12	MIDAf	Cu(pyridine)4(OTf)2	5	47 ± 14

^aStandard conditions: 1 (15 μmol), Cu source (0.75 μmol), Na[¹²⁵I] solution (3–6 MBq) in 150 μL of MeOH:ACN (4:1).

^bRCC determined by radio-HPLC (average of n = ≥ 2 runs). Identity of the product was determined by HPLC using 4-iodoanisole as the reference standard.

^cNC = No Conversion.

^dNo substrate was used.

^eBnep = boronic acid neopentylglycol ester.

^fMIDA = N-methyliminodiacetic acid.

Optimized conditions were then applied to radiodination and astatination of aryl boronic esters with varying functional groups (Scheme 2). The HPLC chromatogram of each respective non-radioactive ¹²⁷I standard was used to identify both the ¹²⁵I- and ²¹¹At-labelled products due to the near identical retention times of the radiolabeled congeners resulting from the chemical similarities of iodine and astatine (see Supporting Information).

Scheme 2. Scope of ¹²⁵I- and ²¹¹At-Radiolabeling of Boronic Esters^a.

^aStandard conditions: 1 (15 μmol), Cu(pyridine)₄(OTf)₂ (0.75 μmol), Na[¹²⁵I] or Na[²¹¹At] solution (3–6 MBq) in 150 μL of MeOH:ACN (4:1). RCC for ¹²⁵I- and ²¹¹At-labeled compounds were each determined by radio-TLC (n = 3). ^bRCC for ¹²⁵I- and ²¹¹At-labeled compounds were each determined by radio-HPLC (n = 3).

We found reaction conditions to be well tolerant of both electron-rich and -poor substrates, 3a–c, including sterically crowded substrates 3b–c, which have been reported to slow down the transmetallation step in Cu-catalyzed radiolabeling protocols.^{17, 21, 22} Excellent ¹²⁵I and ²¹¹At RCCs were also obtained with morpholine and piperazine substrates, 3i–j, prevalent nitrogen heterocycles found in many bioactive compounds and FDA-approved drugs.²⁶ Quantitative ¹²⁵I and ²¹¹At RCC was observed with 3k, providing access to ¹²⁵I and ²¹¹At-labelled benzoate N-hydroxysuccinimide esters, compounds commonly employed in radiolabeling proteins.^{1, 27, 28} Good RCCs continued with nitrogen and sulfur heterocyles, including 5-membered ring systems as well (4l–p).

We next examined the efficiency of the method to radiolabel molecules with ¹²⁵I and ²¹¹At that have potential clinical applications (Scheme 3). Thus, we applied this radiolabeling approach to Poly (ADP-ribose) polymerase inhibitors (PARPi), effective cancer therapeutics for patients with BRCA mutations,²⁹ a current research focus in our laboratory. However, using the optimized conditions with PARPi precursors 5a–b, we only achieved RCYs of 61% and 69%, respectively. When utilizing a 1:1 ratio of Cu(pyridine)₄(OTf)₂ and electron-rich ligand 3,4,7,8-tetramethyl-1,10-phenanthroline, we obtained excellent RCCs and RCYs of 6a–b.

Scheme 3. Access to ¹²⁵I- and ²¹¹At-PARP-1 Inhibitor Compounds via Boronic Ester Precursor 5a–b^a.

^aStandard conditions: 1 (15 μmol), Cu(pyridine)₄(OTf)₂ (0.75 μmol), 3,4,7,8-tetramethyl-1,10-phenanthroline (0.75 μmol), Na[¹²⁵I] or Na[²¹¹At] solution (3–6 MBq) in 150 μL of MeOH:ACN (4:1). RCC and RCY for ¹²⁵I- and ²¹¹At-labeled compounds were each determined by radio-HPLC (n = 3).

The RCYs of 6b obtained using our catalytic approach rival those in previous reports, outlined in Figure 1, which require reaction temperatures of 100 °C or greater,^{30, 31} or 2 to 4 hour reaction times.^{32, 33} Although the astatinated analogue of 6b has been disclosed by Reiner and co-workers³⁴ (no RCY reported), they reported a three-hour synthetic procedure with ²¹¹At, an unusual approach considering the 7.2 hour half-life of the radionuclide.

Figure 1.

Current synthetic radiolabeling routes of 6b

Finally, we prepared 5c to access our previously reported [¹²⁵I]KX1³⁵ and [²¹¹At]MM4 using our Cu/ligand protocol (Scheme 4). Compared to tin-precursor 5c′, we were able to obtain greater RCCs and RCYs using 5c with ¹²⁵I and ²¹¹At, at much milder reaction conditions.

Scheme 4. Access to ¹²⁵I- and ²¹¹At-PARP-1 Inhibitor Compounds via Boronic Ester Precursor 5c^a.

^aStandard conditions: 1 (15 μmol), Cu(pyridine)₄(OTf)₂ (0.75 μmol), 3,4,7,8-tetramethyl-1,10-phenanthroline (0.75 μmol), Na[¹²⁵I] or Na[²¹¹At] solution (3–6 MBq) in 150 μL of MeOH:ACN (4:1). RCC and RCY for ¹²⁵I- and ²¹¹At-labeled compounds were each determined by radio-HPLC (n = 3).

We have demonstrated the first approach to astatinated compounds using boronic ester precursors, providing an efficient and non-toxic protocol that eliminates the traditional need for toxic organotin reagents. This divergent synthesis delivers exceptional radionuclide incorporation for ¹²⁵I and ²¹¹At at room temperature, and is applicable to simple and complex boronic ester precursors, allowing for late-stage radiolabeling with heavy halides. This platform can be applied to other diverse and biologically relevant precursors, allowing for a more practical approach in developing α-emitting therapeutics.