Synthesis and Biological Evaluation of 5'-O-Fatty Acyl Ester Derivatives of 3'-Fluoro-2',3'-dideoxythymidine as Potential Anti-HIV Microbicides.

A number of 5'-O-fatty acyl derivatives of 3'-fluoro-2',3'-dideoxythymidine (FLT, 1) were synthesized. These conjugates were evaluated for their potential as topical microbicides with anti-HIV activity against cell-free (X4 and R5), cell-associated, and multidrug-resistant viruses. Compared to FLT and 3'-azido-2',3'-dideoxythymidine (AZT), 5'-O-(12-azidododecanoyl) (5), 5'-O-myristoyl (6), and 5'-O-(12-thioethyldodecanoyl) (8) derivatives of FLT were found to be more active against both cell-free viruses (lymphocytotropic and monocytotropic strains) with EC50 values of 0.4 µM, 1.1 µM, and <0.2 µM, respectively, as well as cell-associated virus with EC50 values of 12.6, 6.4, and 2.3 µM, respectively. Conjugates 5, 6, and 8 exhibited >4 and >30 times better antiviral index than FLT and AZT, respectively. Conjugates 5 and 8 were significantly more potent than FLT against many multidrug-resistant strains. A comparison of the anti-HIV activity with the corresponding non-hydrolyzable ether conjugates suggested that ester hydrolysis to FLT and fatty acids is critical to enable anti-HIV activity. Cellular uptake studies were conducted using fluorescent derivatives of FLT attached with 5(6)-carboxyfluorescein through either ß-alanine (23) or 12-aminododecanoic acid (24) spacers. The lipophilic fluorescent analog with a long chain (24) showed more than 12 times higher cellular uptake profile than the fluorescent analog with a short chain (23). These studies further confirmed that the attachment of fatty acids improved the cellular uptake of nucleoside conjugates. In addition, 5, 6, and 8 were the least cytotoxic and did not alter vaginal cell and sperm viability compared to the positive control, a commercial topical spermicide (N-9), which significantly decreased sperm and vaginal cell viability inducing the generation of proinflammatory cytokines.

Synthesis and anti-HIV activities of unsymmetrical long chain dicarboxylate esters of dinucleoside reverse transcriptase inhibitors.

A series of 11 unsymmetrical dicarboxylate conjugates of dinucleoside reverse transcriptase inhibitors were synthesized. Three dicarboxylic acids, succinic acid, suberic acid and 1,14-tetradecandioc acid, were diesterified with either 3'-azido-2',3'-dideoxythymidine (AZT), 3'-fluoro-2',3'-dideoxythymidine (FLT), 2',3'-dideoxy-3'-thiacytidine (3TC), or 5-fluoro-2',3'-dideoxy-3'-thiacytidine (FTC). The anti-HIV activity of synthesized compounds was evaluated against HIV-1 X4 (IIIB) and R5 (BaL) viral strains in single-round infection assays. Results indicated that the tetradecandioate esters of nucleosides were more active against HIV than the corresponding parent nucleosides and nucleoside conjugates. The tetradecandioate conjugate of FLT and FTC (5) was found to be the most potent compounds with EC50 values of 47 and 75nM against X4 and R5 HIV-1 strains, respectively, while the EC50 values for the parent analogs, FLT and FTC, ranged from 700 to 3300nM.

Calcium Uncaging with Visible Light.

We have designed a nitroaromatic photochemical protecting group that absorbs visible light in the violet-blue range. The chromophore is a dinitro derivative of bisstyrylthiophene (or BIST) that absorbs light very effectively (ε440 = 66,000 M(-1) cm(-1) and two-photon cross section of 350 GM at 775 nm). We developed a "caged calcium" molecule by conjugation of BIST to a Ca(2+) chelator that upon laser flash photolysis rapidly releases Ca(2+) in <0.2 ms. Using the patch-clamp method the optical probe, loaded with Ca(2+), was delivered into acutely isolated mouse cardiac myocytes, where either one- and two-photon uncaging of Ca(2+) induced highly local or cell-wide physiological Ca(2+) signaling events.

Caged compounds for multichromic optical interrogation of neural systems.

Caged compounds are widely used by neurophysiologists to study many aspects of cellular signaling in glia and neurons. Biologically inert before irradiation, they can be loaded into cells via patch pipette or topically applied in situ to a defined concentration; photolysis releases the caged compound in a very rapid and spatially defined way. As caged compounds are exogenous optical probes, they include not only natural products such neurotransmitters, calcium and IP3 but non-natural products such as fluorophores, drugs and antibodies. In this Technical Spotlight we provide a short introduction to the uncaging technique by discussing the nitroaromatic caging chromophores most widely used in such experiments [e.g. α-carboxy-ortho-nitrobenyl (CNB), dimethoxynitrobenzyl (DMNB), 4-methoxy-7-nitroindolinyl (MNI) and 4-carboxymethoxy-7-nitroindolinyl (CDNI)]. We show that recently developed caging chromophores [rutheniumbipyridial (RuBi) and 7-diethylaminocoumarin (DEAC)450] that are photolyzed with blue light (~ 430-480 nm range) can be combined with traditional nitroaromatic caged compounds to enable two-color optical probing of neuronal function. For example, one-photon uncaging of either RuBi-GABA or DEAC450-GABA with a 473-nm laser is facile, and can block nonlinear currents (dendritic spikes or action potentials) evoked by two-photon uncaging of CDNI-Glu at 720 nm. We also show that two-photon uncaging of DEAC450-Glu and CDNI-GABA at 900 and 720 nm, respectively, can be used to fire and block action potentials. Our experiments illustrate that recently developed chromophores have taken uncaging out of the 'monochrome era', in which it has existed since 1978, so as to enable multichromic interrogation of neuronal function with single-synapse precision.

Synthesis and Biological Evaluation of 5'-O-Dicarboxylic Fatty Acyl Monoester Derivatives of Anti-HIV Nucleoside Reverse Transcriptase Inhibitors.

A number of 5'-O-dicarboxylic fatty acyl monoester derivatives of 3'-azido-3'-deoxythymidine (zidovudine, AZT), 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T), and 3'-fluoro-3'-deoxythymidine (alovudine, FLT) were synthesized to improve the lipophilicity and potentially the cellular delivery of parent polar 2', 3'-dideoxynucleoside (ddN) analogues. The compounds were evaluated for their anti-HIV activity. Three different fatty acids with varying chain length of suberic acid (octanedioic acid), sebacic acid (decanedioic acid), and dodecanedioic acid were used for the conjugation with the nucleosides. The compounds were evaluated for anti-HIV activity and cytotoxicity. All dicarboxylic ester conjugates of nucleosides exhibited significantly higher anti-HIV activity than that of the corresponding parent nucleoside analogs. Among all the tested conjugates, 5'-O-suberate derivative of AZT (EC50 = 0.10 nM) was found to be the most potent compound and showed 80-fold higher anti-HIV activity than AZT without any significant toxicity (TC50 > 500 nM).

Ca 2+ -dependent phosphodiesterase 1 regulates the plasticity of striatal spiny projection neuron glutamatergic synapses.

Long-term synaptic plasticity at glutamatergic synapses on striatal spiny projection neurons (SPNs) is central to learning goal-directed behaviors and habits. Although considerable attention has been paid to the mechanisms underlying synaptic strengthening and new learning, little scrutiny has been given to those involved in the attenuation of synaptic strength that attends suppression of a previously learned association. Our studies revealed a novel, non-Hebbian, long-term, postsynaptic depression of glutamatergic SPN synapses induced by interneuronal nitric oxide (NO) signaling (NO-LTD) that was preferentially engaged at quiescent synapses. This form of plasticity was gated by local Ca 2+ influx through CaV1.3 Ca 2+ channels and stimulation of phosphodiesterase 1 (PDE1), which degraded cyclic guanosine monophosphate (cGMP) and blunted NO signaling. Consistent with this model, mice harboring a gain-of-function mutation in the gene coding for the pore-forming subunit of CaV1.3 channels had elevated depolarization-induced dendritic Ca 2+ entry and impaired NO-LTD. Extracellular uncaging of glutamate and intracellular uncaging of cGMP suggested that this Ca 2+ -dependent regulation of PDE1 activity allowed for local regulation of dendritic NO signaling. This inference was supported by simulation of SPN dendritic integration, which revealed that dendritic spikes engaged PDE1 in a branch-specific manner. In a mouse model of Parkinson's disease (PD), NO-LTD was absent not because of a postsynaptic deficit in NO signaling machinery, but rather due to impaired interneuronal NO release. Re-balancing intrastriatal neuromodulatory signaling in the PD model restored NO release and NO-LTD. Taken together, these studies provide novel insights into the mechanisms governing NO-LTD in SPN and its role in psychomotor disorders, like PD.

Ca2+-dependent phosphodiesterase 1 regulates the plasticity of striatal spiny projection neuron glutamatergic synapses.

Long-term synaptic plasticity at glutamatergic synapses on striatal spiny projection neurons (SPNs) is central to learning goal-directed behaviors and habits. Our studies reveal that SPNs manifest a heterosynaptic, nitric oxide (NO)-dependent form of long-term postsynaptic depression of glutamatergic SPN synapses (NO-LTD) that is preferentially engaged at quiescent synapses. Plasticity is gated by Ca2+ entry through CaV1.3 Ca2+ channels and phosphodiesterase 1 (PDE1) activation, which blunts intracellular cyclic guanosine monophosphate (cGMP) and NO signaling. Both experimental and simulation studies suggest that this Ca2+-dependent regulation of PDE1 activity allows for local regulation of dendritic cGMP signaling. In a mouse model of Parkinson disease (PD), NO-LTD is absent because of impaired interneuronal NO release; re-balancing intrastriatal neuromodulatory signaling restores NO release and NO-LTD. Taken together, these studies provide important insights into the mechanisms governing NO-LTD in SPNs and its role in psychomotor disorders such as PD.

Cellular influx, efflux, and anabolism of 3-carboranyl thymidine analogs: potential boron delivery agents for neutron capture therapy.

3-[5-{2-(2,3-Dihydroxyprop-1-yl)-o-carboran-1-yl}pentan-1-yl]thymidine (N5-2OH) is a first generation 3-carboranyl thymidine analog (3CTA) that has been intensively studied as a boron-10 ((10)B) delivery agent for neutron capture therapy (NCT). N5-2OH is an excellent substrate of thymidine kinase 1 and its favorable biodistribution profile in rodents led to successful preclinical NCT of rats bearing intracerebral RG2 glioma. The present study explored cellular influx and efflux mechanisms of N5-2OH, as well as its intracellular anabolism beyond the monophosphate level. N5-2OH entered cultured human CCRF-CEM cells via passive diffusion, whereas the multidrug resistance-associated protein 4 appeared to be a major mediator of N5-2OH monophosphate efflux. N5-2OH was effectively monophosphorylated in cultured murine L929 [thymidine kinase 1 (TK1(+))] cells whereas formation of N5-2OH monophosphate was markedly lower in L929 (TK1(-)) cell variants. Further metabolism to the di- and triphosphate forms was not observed in any of the cell lines. Regardless of monophosphorylation, parental N5-2OH was the major intracellular component in both TK1(+) and TK1(-) cells. Phosphate transfer experiments with enzyme preparations showed that N5-2OH monophosphate, as well as the monophosphate of a second 3-carboranyl thymidine analog [3-[5-(o-carboran-1-yl)pentan-1-yl]thymidine (N5)], were not substrates of thymidine monophosphate kinase. Surprisingly, N5-diphosphate was phosphorylated by nucleoside diphosphate kinase although N5-triphosphate apparently was not a substrate of DNA polymerase. Our results provide valuable information on the cellular metabolism and pharmacokinetic profile of 3-carboranyl thymidine analogs.

Emtricitabine prodrugs with improved anti-HIV activity and cellular uptake.

Three fatty acyl conjugates of (-)-2',3'-dideoxy-5-fluoro-3'-thiacytidine (FTC, emtricitabine) were synthesized and evaluated against HIV-1 cell-free and cell-associated virus and compared with the corresponding parent nucleoside and physical mixtures of FTC and fatty acids. Among all the compounds, the myristoylated conjugate of FTC (5, EC(50) = 0.07-3.7 µM) displayed the highest potency. Compound 5 exhibited 10-24 and 3-13-times higher anti-HIV activity than FTC alone (EC(50) = 0.7-88.6 µM) and the corresponding physical mixtures of FTC and myristic acid (14, EC(50) = 0.2-20 µM), respectively. Cellular uptake studies confirmed that compound 5 accumulated intracellularly after 1 h of incubation and underwent intracellular hydrolysis in CCRF-CEM cells. Alternative studies were conducted using the carboxyfluorescein conjugated with FTC though ß-alanine (12) and 12-aminododecanoic acid (13). Acylation of FTC with a long-chain fatty acid in 13 improved its cellular uptake by 8.5-20 fold in comparison to 12 with a short-chain ß-alanine. Compound 5 (IC(90) = 15.7-16.1 nM) showed 6.6- and 35.2 times higher activity than FTC (IC(90) = 103-567 nM) against multidrug resistant viruses B-NNRTI and B-K65R, indicating that FTC conjugation with myristic acid generates a more potent analogue with a better resistance profile than its parent compound.

Synthesis and anti-HIV activities of symmetrical dicarboxylate esters of dinucleoside reverse transcriptase inhibitors.

Three nucleoside analogues, 3'-fluoro-2',3'-dideoxythymidine (FLT), 3'-azido-2',3'-dideoxythymidine (AZT), and 2',3'-dideoxy-3'-thiacytidine (3TC) were conjugated with three different dicarboxylic acids to afford the long chain dicarboxylate esters of nucleosides. In general, dinucleoside ester conjugates of FLT and 3TC with long chain dicarboxylic acids exhibited higher anti-HIV activity than their parent nucleosides. Dodecanoate and tetradecanoate dinucleoside ester derivatives of FLT were found to be the most potent compounds with EC(50) values of 0.8-1.0 nM and 3-4 nM against HIV-1(US/92/727) and HIV-1(IIIB) cells, respectively. The anti-HIV activity of the 3TC conjugates containing long chain dicarboxylate diester (EC(50)=3-60 nM) was improved by 1.5-66 fold when compared to 3TC (EC(50)=90-200 nM). This study reveals that the symmetrical ester conjugation of dicarboxylic acids with a number of nucleosides results in conjugates with improved anti-HIV profile.

Synthesis, biological evaluation, and radioiodination of halogenated closo-carboranylthymidine analogues.

The synthesis and initial biological evaluation of 3-carboranylthymidine analogues (3CTAs) that are (radio)halogenated at the closo-carborane cluster are described. Radiohalogenated 3CTAs have the potential to be used in the radiotherapy and imaging of cancer because they may be selectively entrapped in tumor cells through monophosphorylation by human thymidine kinase 1 (hTK1). Two strategies for the synthesis of a (127)I-labeled form of a specific 3CTA, previously designated as N5, are described: (1) direct iodination of N5 with iodine monochloride and aluminum chloride to obtain N5-(127)I and (2) initial monoiodination of o-carborane to 9-iodo-o-carborane followed by its functionalization to N5-(127)I. The former strategy produced N5-(127)I in low yields along with di-, tri-, and tetraiodinated N5 as well as decomposition products, whereas the latter method produced only N5-(127)I in high yields. N5-(127)I was subjected to nucleophilic halogen- and isotope-exchange reactions using Na(79/81)Br and Na(125)I, respectively, in the presence of Herrmann's catalyst to obtain N5-(79/81)Br and N5-(125)I, respectively. Two intermediate products formed using the second strategy, 1-(tert-butyldimethylsilyl)-9-iodo-o-carborane and 1-(tert-butyldimethylsilyl)-12-iodo-o-carborane, were subjected to X-ray diffraction studies to confirm that substitution at a single carbon atom of 9-iodo-o-carborane resulted in the formation of two structural isomers. To the best of our knowledge, this is the first report of halogen- and isotope-exchange reactions of B-halocarboranes that have been conjugated to a complex biomolecule. Human TK1 phosphorylation rates of N5, N5-(127)I, and N5-(79/81)Br ranged from 38.0% to 29.6% relative to that of thymidine, the endogenous hTK1 substrate. The in vitro uptake of N5, N5-(127)I, and N5-(79/81)Br in L929 TK1(+) cells was 2.0, 1.8, and 1.4 times greater than that in L929 TK1(-) cells.

The yin and yang of intracellular delivery of amphipathic optical probes using n-butyl charge masking.

Monitoring and manipulation of ionized intracellular calcium concentrations within intact, living cells using optical probes with organic chromophores is a core method for cell physiology. Since all these probes have multiple negative charges, they must be smuggled through the plasma membrane in a transiently neutral form, with intracellular esterases used to deprotect the masked anions. Here we explore the ability of the synthetically easily accessible n-butyl ester protecting group to deliver amphipathic cargoes to the cytosol. We show that the size of the caging chromophore conditions the ability of intracellular probe delivery and esterase charge unmasking.

Synthesis and biological evaluation of fatty acyl ester derivatives of 2',3'-didehydro-2',3'-dideoxythymidine.

A number of 5'-O-fatty acyl derivatives of 2',3'-didehydro-2',3'-dideoxythymidine (stavudine, d4T) were synthesized and evaluated for anti-HIV activities against cell-free and cell-associated virus, cellular cytotoxicity, and cellular uptake studies. The conjugates were found to be more potent than d4T. Among these conjugates, 5'-O-12-azidododecanoyl derivative of d4T (2), displaying EC(50) = 3.1-22.4 µM, showed 4- to 9-fold higher activities than d4T against cell-free and cell-associated virus. Cellular uptake studies were conducted on CCRF-CEM cell line using 5(6)-carboxyfluorescein derivatives of d4T attached through ß-alanine (9) or 12-aminododecanoic acid (10) as linkers. The fluorescein-substituted analog of d4T with long chain length (10) showed 12- to 15-fold higher cellular uptake profile than the corresponding analog with short chain length (9). These studies reveal that conjugation of fatty acids to d4T enhances the cellular uptake and anti-HIV activity of stavudine.

Synthesis of Closo-1,7-Carboranyl Alkyl Amines.

Of the three closo-carborane isomers (C(2)B(10)H(12)), closo-1,2-carborane has been used most widely in the synthesis of carboranyl amines. However, closo-1,2-carboranes are prone to deboronation to nido-7,8-carborane under various conditions including attack by basic amino groups. In order to overcome this problem, closo-1,7-carboranyl ethyl-, propyl-, and butylamine were synthesized, which should be more stable towards basic deboronation than their closo-1,2-carboranyl counterparts. These closo-1,7-carboranyl amines (5, 18 and 19) were synthesized using two different methods, both starting from the corresponding closo-1,7-carboranyl alkyl iodides (3, 14 and 15). One of the carboranyl alkyl amine (5) was conjugated with folic acid to form a closo-1,7-carborane-folic acid bioconjugate (20).

Synthesis, antiviral and contraceptive activities of nucleoside-sodium cellulose sulfate acetate and succinate conjugates.

Chemical conjugates between sodium cellulose sulfate (CS), displaying contraceptive and HIV-entry inhibiting properties, and nucleoside reverse transcriptase inhibitors (NRTIs) (3'-azido-2',3'-dideoxythymidine (AZT), 3'-fluoro-2',3'-dideoxythymidine (FLT), or 2',3'-dideoxy-3'-thiacytidine (3TC)) were designed to simultaneously provide contraceptive and anti-HIV activity. Two linkers, acetate and succinate, were used to conjugate the nucleoside analogs with CS. The conjugates containing cellulose sulfate-acetate (CSA) (e.g., AZT-CSA and FLT-CSA) were found to be more potent than CS and other conjugates (e.g., AZT-succinate-CS, and FLT-succinate-CS). The presence of both sulfate and the acetate groups on cellulose were critical for generating maximum anti-HIV activity. In addition to showing equal potency against wild-type and multidrug resistant HIV-1, the AZT-CSA conjugate displayed significant contraceptive activity in an animal model, providing the initial proof-of-concept for the design and synthesis of dual-activity compounds based on these combinations.

Intracellular Uncaging of cGMP with Blue Light.

We have made a new caged cGMP that is photolyzed with blue light. Using our recently developed derivative of 7-diethylaminocourmarin (DEAC) called DEAC450, we synthesized coumarin phosphoester derivatives of cGMP with two negative charges appended to the DEAC450 moiety. DEAC450-cGMP is freely soluble in physiological buffer without the need for any organic cosolvents. With a photolysis quantum yield of 0.18 and an extinction coefficient of 43 000 M-1 cm-1 at 453 nm, DEAC450-cGMP is the most photosensitive caged cGMP made to date. In patch-clamped neurons in acutely isolated brain slices, blue light effectively uncaged cGMP from DEAC450 and facilitated activation of hyperpolarization and cyclic nucleotide gated cation (HCN) channels in cholinergic interneurons. Thus, DEAC450-cGMP has a unique set of optical and chemical properties that make it a useful addition to the optical arsenal available to neurobiologists.

Synthesis and evaluation of thymidine kinase 1-targeting carboranyl pyrimidine nucleoside analogs for boron neutron capture therapy of cancer.

A library of sixteen 2nd generation amino- and amido-substituted carboranyl pyrimidine nucleoside analogs, designed as substrates and inhibitors of thymidine kinase 1 (TK1) for potential use in boron neutron capture therapy (BNCT) of cancer, was synthesized and evaluated in enzyme kinetic-, enzyme inhibition-, metabolomic-, and biodistribution studies. One of these 2nd generation carboranyl pyrimidine nucleoside analogs (YB18A [3]), having an amino group directly attached to a meta-carborane cage tethered via ethylene spacer to the 3-position of thymidine, was approximately 3-4 times superior as a substrate and inhibitor of hTK1 than N5-2OH (2), a 1st generation carboranyl pyrimidine nucleoside analog. Both 2 and 3 appeared to be 5'-monophosphorylated in TK1(+) RG2 cells, both in vitro and in vivo. Biodistribution studies in rats bearing intracerebral RG2 glioma resulted in selective tumor uptake of 3 with an intratumoral concentration that was approximately 4 times higher than that of 2. The obtained results significantly advance the understanding of the binding interactions between TK1 and carboranyl pyrimidine nucleoside analogs and will profoundly impact future design strategies for these agents.

Design, synthesis, antiviral activity, and pre-formulation development of poly-L-arginine-fatty acyl derivatives of nucleoside reverse transcriptase inhibitors.

The objective of this work was to design conjugates of anti-HIV nucleosides conjugated with fatty acids and cell-penetrating poly-L-arginine (polyArg) peptides. Three conjugates of polyArg cell-penetrating peptides with fatty acyl derivatives of alovudine (FLT), lamivudine (3TC), and emtricitabine (FTC) were synthesized. In general, the compounds exhibited anti-HIV activity against X4 and R5 cell-free virus with EC50 values of 1.5-16.6 µM. FLT-CO-(CH2)12-CO-(Arg)7 exhibited EC50 values of 2.9 µM and 3.1 µM against X4 and R5 cell-free virus, respectively. The FLT conjugate was selected for further preformulation studies by determination of solution state degradation and lipid solubility. The compound was found to be stable in neutral and oxidative conditions and moderately stable in heated conditions.

Nucleoside reverse transcriptase inhibitors possess intrinsic anti-inflammatory activity.

Nucleoside reverse transcriptase inhibitors (NRTIs) are mainstay therapeutics for HIV that block retrovirus replication. Alu (an endogenous retroelement that also requires reverse transcriptase for its life cycle)-derived RNAs activate P2X7 and the NLRP3 inflammasome to cause cell death of the retinal pigment epithelium in geographic atrophy, a type of age-related macular degeneration. We found that NRTIs inhibit P2X7-mediated NLRP3 inflammasome activation independent of reverse transcriptase inhibition. Multiple approved and clinically relevant NRTIs prevented caspase-1 activation, the effector of the NLRP3 inflammasome, induced by Alu RNA. NRTIs were efficacious in mouse models of geographic atrophy, choroidal neovascularization, graft-versus-host disease, and sterile liver inflammation. Our findings suggest that NRTIs are ripe for drug repurposing in P2X7-driven diseases.

Synthesis of N3-substituted carboranyl thymidine bioconjugates and their evaluation as substrates of recombinant human thymidine kinase 1.

Four different libraries of overall twenty three N3-substituted thymidine (dThd) analogues, including eleven 3-carboranyl thymidine analogues (3CTAs), were synthesized. The latter are potential agents for Boron Neutron Capture Therapy (BNCT) of cancer. Linker between the dThd scaffold and the m-carborane cluster at the N3-position of the 3CTAs contained amidinyl-(3e and 3f), guanidyl-(7e-7g), tetrazolylmethyl-(9b1/2-9d1/2), or tetrazolyl groups (11b1/2-11d1/2) to improve human thymidine kinase 1 (hTK1) substrate characteristics and water solubilities compared with 1st generation 3CTAs, such as N5 and N5-2OH. The amidinyl- and guanidyl-type N3-substitued dThd analogues (3a-3f and 7a-7g) had hTK1 phosphorylation rates of <30% relative to that of dThd, the endogenous hTK1 substrate, whereas the tetrazolyl-type N3-substitued dThd analogues (9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2) had relative phosphorylation rates (rPRs) of >40%. Compounds 9a, 9b1/2-9d1/2 and 11a, 11b1/2-11d1/2 were subjected to in-depth enzyme kinetics studies and the obtained rk(cat)/K(m) (k(cat)/K(m) relative to that of dThd) ranged from 2.5 to 26%. The tetrazolyl-type N3-substitued dThd analogues 9b1/2 and 11d1/2 were the best substrates of hTK1 with rPRs of 52.4% and 42.5% and rk(cat)/K(m) values of 14.9% and 19.7% respectively. In comparison, the rPR and rk(cat)/K(m) values of N5-2OH in this specific study were 41.5% and 10.8%, respectively. Compounds 3e and 3f were >1900 and >1500 times, respectively, better soluble in PBS (pH 7.4) than N5-2OH whereas solubilities for 9b1/2-9d1/2 and 11b1/2-11d1/2 were only 1.3-13 times better.