Biochemical pharmacology and toxicology of 8-azaadenosine alone and in combination with 2'-deoxycoformycin (pentostatin)

Nuclear aggregates of NONO/SFPQ and A-to-I-edited RNA in Parkinson's disease and dementia with Lewy bodies

Neurodegenerative diseases are commonly classified as proteinopathies that are defined by the aggregation of a specific protein. Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are classified as synucleinopathies since α-synuclein (α-syn)-containing inclusions histopathologically define these diseases. Unbiased biochemical analysis of PD and DLB patient material unexpectedly revealed novel pathological inclusions in the nucleus comprising adenosine-to-inosine (A-to-I)-edited mRNAs and NONO and SFPQ proteins. These inclusions showed no colocalization with Lewy bodies and accumulated at levels comparable to α-syn. NONO and SFPQ aggregates reduced the expression of the editing inhibitor ADAR3, increasing A-to-I editing mainly within human-specific, Alu-repeat regions of axon, synaptic, and mitochondrial transcripts. Inosine-containing transcripts aberrantly accumulated in the nucleus, bound tighter to recombinant purified SFPQ in vitro, and potentiated SFPQ aggregation in human dopamine neurons, resulting in a self-propagating pathological state. Our data offer new insight into the inclusion composition and pathophysiology of PD and DLB.

8-azaadenosine and 8-chloroadenosine are not selective inhibitors of ADAR

The RNA editing enzyme ADAR, is an attractive therapeutic target for multiple cancers. Through its deaminase activity, ADAR edits adenosine to inosine in dsRNAs. Loss of ADAR in some cancer cell lines causes activation of the type I interferon pathway and the PKR translational repressor, leading to inhibition of proliferation and stimulation of cell death. As such, inhibition of ADAR function is a viable therapeutic strategy for many cancers. However, there are no FDA approved inhibitors of ADAR. Two small molecules have been previously shown to inhibit ADAR or reduce its expression: 8-azaadenosine and 8-chloroadenosine. Here we show that neither molecule is a selective inhibitor of ADAR. Both 8-azaadenosine and 8-chloroadenosine show similar toxicity to ADAR-dependent and independent cancer cell lines. Furthermore, the toxicity of both small molecules is comparable between cell lines with either knockdown or overexpression of ADAR, and cells with unperturbed ADAR expression. Treatment with neither molecule causes activation of PKR. Finally, treatment with either molecule has no effect on A-to-I editing of multiple ADAR substrates. Together these data show that 8-azaadenosine and 8-chloroadenosine are not suitable small molecules for therapies that require selective inhibition of ADAR, and neither should be used in preclinical studies as ADAR inhibitors.

pH Controlled Intersystem Crossing and Singlet Oxygen Generation of 8-Azaadenine in Aqueous Solution

Azabases are intriguing DNA and RNA analogues and have been used as effective antiviral and anticancer medicines. However, photosensitivity of these drugs has also been reported. Here, pH-controlled intersystem crossing (ISC) process of 9H 8-azaadenine (8-AA) in aqueous solution is reported. Broadband transient absorption measurements reveal that the hydrogen atom at N9 position can greatly affect ISC of 8-AA and ISC is more favorable when 8-AA is in its neutral form in aqueous solution. The initial excited ππ* (S₂ ) state evolves through ultrafast internal conversion (IC) (4.2 ps) to the lower-lying nπ* state (S₁ ), which further stands as a door way state for ISC with a time constant of 160 ps. The triplet state has a lifetime of 6.1 μs. On the other hand, deprotonation at N9 position promotes the IC from the ππ* (S₂ ) state to the ground state (S₀ ) and the lifetime of the S₂ state is determined to be 10 ps. The experimental results are further supported by time-dependent density functional theory (TDDFT) calculations. Singlet oxygen generation yield is measured to be 13.8 % for the neutral 8-AA while the deprotonated one exhibit much lower yield (<2 %), implying that this compound could be a potential pH-sensitized photodynamic therapy agent.

Effects of adenosine analogues on ATP concentrations in human erythrocytes. Further evidence for a route independent of adenosine kinase

Adenosine derivatives are frequently used in chemotherapy because of their potent antitumor, antiviral and antiparasitic activity. We investigated the metabolism of some adenosine analogues in adenosine deaminase inhibited normal and adenine phosphoribosyltransferase (APRT) deficient human erythrocytes. The ATP and GTP concentrations and the formation of unusual nucleotides were measured. Some of the analogues studied (tubercidin, 9 beta-D-arabinofuranosyladenine, 2'-deoxyadenosine, 2-chloroadenosine, neplanocin A) were phosphorylated to the corresponding nucleoside triphosphates and this process was abolished by iodotubercidin--an adenosine kinase inhibitor. With the exception of 2'-deoxyadenosine, nucleotide analogue formation was accompanied by ATP depletion. ATP decrease was not observed after adenosine kinase inhibition and ATP concentration even increased in the presence of 2'-deoxyadenosine, neplanocin A and 5'-iodo-5'-deoxyadenosine. However, the latter increment was not observed in APRT deficient erythrocytes. Bredinin, S-adenosylhomocysteine, deoxycoformycin and adenosine dialdehyde did not form nucleotide derivatives or exert any effects on ATP concentration. It is concluded that adenosine analogues can either enter the nucleotide pool via phosphorylation mechanisms, or may be converted to ATP by the pathways involving the intermediate formation of adenine.

Transport of deoxycoformycin in human erythrocytes. Measurement by adenosine deaminase titration and radioisotope assays

The assay of residual adenosine deaminase (ADA) activity was used as a sensitive measure of the transport of deoxycoformycin (dCF) into human erythrocytes. Contrary to prior reports from this laboratory, the inactivation of intraerythrocytic ADA by dCF was linear rather than log-linear, with time. Linear inactivation rates were also seen when erythrocytes were preloaded with a 5-fold excess of calf intestinal ADA. The uptake of tritium-labeled dCF molecules and the rate of inactivation of ADA molecules showed an approximate 1:1 stoichiometry. The nucleoside transport inhibitors, 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine (NBMPR) and dipyridamole, and the permeant, uridine, inhibited dCF transport with Ki values of 35 nM, 45 nM, and 340 microM respectively. The affinity of dCF for the nucleoside transporter was low with a Ki of approximately 10 mM for the inhibition of adenosine influx.