A bifunctional curcumin analogue for two-photon imaging and inhibiting crosslinking of amyloid beta
In this report, we designed a highly bright bifunctional curcumin analogue CRANAD-28. In vivo two-photon imaging suggested that CRANAD-28 could penetrate the blood brain barrier (BBB) and label plaques and cerebral amyloid angiopathies (CAAs). We also demonstrated that this imaging probe could inhibit the crosslinking of amyloid
beta induced either by copper or by natural conditions. An imaging probe and a drug could share a common structural scaffold and design concept considering that they have the same target.1,2 They could also have similar interaction mechanisms and specificity requirements for the target. In traditional approaches,imaging probes and drugs are pursued separately, which are timeconsuming and costly.3,4 To overcome this disadvantage, developing agents that have potentials for therapy and imaging (theranostic) is highly desirable. Nanoparticle-based theranostics have been intensively explored;4 however, their poor BBB penetration limits their applications in CNS diseases. Therefore, developing small
molecule-based theranostics is warranted for neurodegenerative diseases such as Alzheimer’s disease2 (AD).
Amyloid beta (Ab), a peptide of 40 or 42 amino acids, is one of the key players in AD pathology.5 The presence of Ab plaques is one of the hallmarks of AD.5,6 While the degree of toxicity of amyloid plaques that contribute to the overall cognitive decline in AD remains unknown, plaques are clear sites of pathology as evidenced by the presence of dystrophic neurites and the loss of dendritic spines in their surroundings.7,8 Furthermore, there is also some evidence that plaques may impair mitochondrial function and calcium homeostasis and lead to cell death.9 Given these and other lines of evidence, reducing Ab deposits and plaques remains an important aim for AD drug development.5,10,11 Imaging probes for Abs for both clinical and preclinical research have been reported.10,12–15 However, small molecules that can be used both for imaging and therapy of AD are still urgently needed.11
Recently, we have developed curcumin analogues for detecting soluble and insoluble Abs in vitro, and some of them have been successfully applied for in vivo near-infrared imaging in transgenic AD mice.16–18 Based on the limited interaction mechanism of the curcumin ligands and Abs, we also designed imidazole-containing curcumin analogues to specifically interrupt the crosslinking of Abs that was initialized by metal ions such as copper.19,20 However, curcumin compounds always have low quantum yield (QY), and lack theranostic properties. In this report, we designed, synthesized and tested a bifunctional compound with high brightness for two-photon imaging and potential therapy.
To overcome the low QY limitation of curcumin analogues, we hypothesized that replacing the phenyl rings with pyrazoles could increase the brightness. Conceivably, the inductive electronwithdrawing effect of one of the nitrogens of pyrazole could lead to a low tendency of electron delocalization in the system, which will decrease tautomerization of the designed compounds. It is well known that less tautomerization can reduce non-radiative decay from the excited states, thus increasing the QY.21 On the other hand, in our previous studies,17 we showed that imidazole containing curcumin analogues could specifically interfere with the coordination of copper with H13 and H14 (Histidine) and thus attenuate the crosslinking of Ab. In this report, we speculated that pyrazole could also interfere with the coordination because pyrazole can coordinate with copper as well. In addition, we reasoned that phenyl substitution at the N-1 position of pyrazole could further improve the QY due to the reduction of tautomerization of pyrazole (more tautomers, more non-radiative decay).22 Taking all the facts into consideration, CRANAD-28 was designed and synthesized (Fig. 1b).
CRANAD-28, an orange powder, was synthesized following our previously published procedures.16,17 We first investigated its fluorescence properties such as excitation and emission, as