GS-9219—A Novel Acyclic Nucleotide Analogue with Potent Antineoplastic Activity in Dogs with Spontaneous Non–Hodgkin's Lymphoma

Unachieved antiviral strategies with acyclic nucleoside phosphonates: Dedicated to the memory of dr. Salvatore "Sam" Joseph Enna

Many acyclic nucleoside phosphonates such as cidofovir, adefovir dipivoxil, tenofovir disoproxil fumarate, and tenofovir alafenamide have been marketed for the treatment or prophylaxis of infectious diseases. Here, this review highlights potent acyclic nucleoside phosphonates for their potential in the treatment of retrovirus (e.g., human immunodeficiency virus) and DNA virus (e.g., adeno-, papilloma-, herpes- and poxvirus) infections. If properly assessed and/or optimized, some potent acyclic nucleoside phosphonates can be possibly applied in the control of current and emerging infectious diseases.

Alternating rabacfosadine and doxorubicin for treatment of naïve canine lymphoma

The current standard of care treatment for canine lymphoma is a multi-agent, CHOP-based chemotherapy protocol. Single agent doxorubicin (DOX) is less burdensome; however, multi-agent chemotherapy protocols are often superior. The recently approved drug rabacfosadine (RAB, Tanovea) provides an attractive option for combination therapy with DOX, as both drugs demonstrate efficacy against lymphoma and possess different mechanisms of action. A previous study evaluating alternating RAB/DOX reported an overall response rate (ORR) of 84%, with a median progression-free survival time (PFS) of 194 days. The aim of this prospective trial was to evaluate the same protocol in an additional population of dogs. Fifty-nine dogs with treatment naïve lymphoma were enrolled. RAB (1.0 mg/kg IV) was alternated with DOX (30 mg/m2 IV) every 21 days for up to six total treatments (3 cycles). Response assessment and adverse event (AE) evaluation were performed every 21 days using VCOG criteria. The ORR was 93% (79% CR, 14% PR). The median time to maximal response was 21.5 days; median PFS was 199 days. T cell immunophenotype and lack of treatment response were predictive of inferior outcomes. AEs were mostly gastrointestinal. Six dogs developed presumed or confirmed pulmonary fibrosis; four were grade 5. One dog experienced grade 3 extravasation injury with RAB that resolved with supportive treatment. These data mirror those of the previously reported RAB/DOX study, and support the finding that alternating RAB/DOX is a reasonable treatment option for canine lymphoma.

Novel Treatments for Lymphoma

Lymphoma is a common disease in companion animals. While conventional chemotherapy has the potential to induce remission and prolong life, relapse is common and novel treatments are needed to improve outcome. This review discusses recent modifications/adjustments to conventional standard of care therapy for canine and feline lymphoma, options for treatment or relapsed/refractory disease, and cutting-edge immunotherapy and small molecule-based approaches that are in varying stages of regulatory approval.

Prodrugs of a 1-Hydroxy-2-oxopiperidin-3-yl Phosphonate Enolase Inhibitor for the Treatment of ENO1-Deleted Cancers

Cancers harboring homozygous deletion of the glycolytic enzyme enolase 1 (ENO1) are selectively vulnerable to inhibition of the paralogous isoform, enolase 2 (ENO2). A previous work described the sustained tumor regression activities of a substrate-competitive phosphonate inhibitor of ENO2, 1-hydroxy-2-oxopiperidin-3-yl phosphonate (HEX) (5), and its bis-pivaloyoxymethyl prodrug, POMHEX (6), in an ENO1-deleted intracranial orthotopic xenograft model of glioblastoma [Nature Metabolism 2020, 2, 1423-1426]. Due to poor pharmacokinetics of bis-ester prodrugs, this study was undertaken to identify potential non-esterase prodrugs for further development. Whereas phosphonoamidate esters were efficiently bioactivated in ENO1-deleted glioma cells, McGuigan prodrugs were not. Other strategies, including cycloSal and lipid prodrugs of 5, exhibited low micromolar IC₅₀ values in ENO1-deleted glioma cells and improved stability in human serum over 6. The activity of select prodrugs was also probed using the NCI-60 cell line screen, supporting its use to examine the relationship between prodrugs and cell line-dependent bioactivation.

Phosphoryl Prodrugs: Characteristics to Improve Drug Development

Phosphoryl prodrugs are key compounds in drug development. Biologically active phosphoryl compounds often have negative charges on the phosphoryl group, and as a result, frequently have poor pharmacokinetic (PK) profiles. The use of lipophilic moieties bonded to the phosphorus (or attached oxygen atoms) masks the negative charge of the phosphoryl group, cleavage releasing the active molecule. The use of prodrugs to improve the PK of active parent molecules is an essential step in drug development. This review highlights promising trends in terminal elimination half-life, C_max, clearance, oral bioavailability, and cLogP in phosphoryl prodrugs. We focus on specific prodrug families: esters, amidates, and ProTides. We conclude that moderating lipophilicity is a key part of prodrug success. This type of evaluation is important for drug development, regardless of clinical application. It is our hope that this analysis, and future ones like it, will play a significant role in prodrug evolution.

Phosphonates and Phosphonate Prodrugs in Medicinal Chemistry: Past Successes and Future Prospects

Compounds with a phosphonate group, i.e., -P(O)(OH)₂ group attached directly to the molecule via a P-C bond serve as suitable non-hydrolyzable phosphate mimics in various biomedical applications. In principle, they often inhibit enzymes utilizing various phosphates as substrates. In this review we focus mainly on biologically active phosphonates that originated from our institute (Institute of Organic Chemistry and Biochemistry in Prague); i.e., acyclic nucleoside phosphonates (ANPs, e.g., adefovir, tenofovir, and cidofovir) and derivatives of non-nucleoside phosphonates such as 2-(phosphonomethyl) pentanedioic acid (2-PMPA). Principal strategies of their syntheses and modifications to prodrugs is reported. Besides clinically used ANP antivirals, a special attention is paid to new biologically active molecules with respect to emerging infections and arising resistance of many pathogens against standard treatments. These new structures include 2,4-diamino-6-[2-(phosphonomethoxy)ethoxy]pyrimidines or so-called "open-ring" derivatives, acyclic nucleoside phosphonates with 5-azacytosine as a base moiety, side-chain fluorinated ANPs, aza/deazapurine ANPs. When transformed into an appropriate prodrug by derivatizing their charged functionalities, all these compounds show promising potential to become drug candidates for the treatment of viral infections. ANP prodrugs with suitable pharmacokinetics include amino acid phosphoramidates, pivaloyloxymethyl (POM) and isopropoxycarbonyloxymethyl (POC) esters, alkyl and alkoxyalkyl esters, salicylic esters, (methyl-2-oxo-1,3-dioxol-4-yl) methyl (ODOL) esters and peptidomimetic prodrugs. We also focus on the story of cytostatics related to 9-[2-(phosphonomethoxy)ethyl]guanine and its prodrugs which eventually led to development of the veterinary drug rabacfosadine. Various new ANP structures are also currently investigated as antiparasitics, especially antimalarial agents e.g., guanine and hypoxanthine derivatives with 2-(phosphonoethoxy)ethyl moiety, their thia-analogues and N-branched derivatives. In addition to ANPs and their analogs, we also describe prodrugs of 2-(phosphonomethyl)pentanedioic acid (2-PMPA), a potent inhibitor of the enzyme glutamate carboxypeptidase II (GCPII), also known as prostate-specific membrane antigen (PSMA). Glutamate carboxypeptidase II inhibitors, including 2-PMPA have been found efficacious in various preclinical models of neurological disorders which are caused by glutamatergic excitotoxicity. Unfortunately its highly polar character and hence low bioavailability severely limits its potential for clinical use. To overcome this problem, various prodrug strategies have been used to mask carboxylates and/or phosphonate functionalities with pivaloyloxymethyl, POC, ODOL and alkyl esters. Chemistry and biological characterization led to identification of prodrugs with 44-80 fold greater oral bioavailability (tetra-ODOL-2-PMPA).

Multicenter, randomized, double‐blinded, placebo‐controlled study of rabacfosadine in dogs with lymphoma

Background

Rabacfosadine (RAB, Tanovea‐CA1) is a novel chemotherapy agent conditionally approved for the treatment of lymphoma in dogs.

Hypothesis/Objectives

To determine the efficacy and safety of RAB in dogs with lymphoma.

Animals

One hundred and fifty‐eight client‐owned dogs with naïve or relapsed multicentric lymphoma were prospectively enrolled from January to October 2019.

Methods

Dogs were randomized to receive RAB or placebo at a 3 : 1 ratio. Treatment was given every 21 days for up to 5 treatments. Study endpoints included progression‐free survival (PFS), overall response rate (ORR) at a given visit, best overall response rate (BORR), and percent progression free 1 month after treatment completion. Safety data were also collected.

Results

The median PFS was significantly longer in the RAB group compared to placebo (82 vs 21 days; P < .0001, HR 6.265 [95% CI 3.947‐9.945]). The BORR for RAB‐treated dogs was 73.2% (50.9% complete response [CR], 22.3% partial response [PR]) and 5.6% (0% CR, 5.6% PR) for placebo‐treated dogs (P < .0001). One month after the last treatment, 37 RAB‐treated dogs (33%) were progression free compared with no placebo‐treated dogs (P < .0001). The most common adverse events observed in the RAB group were diarrhea (87.5%), decreased appetite (68.3%), and vomiting (68.3%) and were generally low grade and reversible. Serious adverse events were reported in 24 RAB‐treated (20%) and 5 placebo‐treated dogs (13%).

Conclusions and Clinical Importance

Rabacfosadine demonstrated statistically significant antitumor efficacy in dogs with lymphoma when administered every 21 days for up to 5 treatments as compared to placebo.

Drug Discovery of Nucleos(t)ide Antiviral Agents: Dedicated to Prof. Dr. Erik De Clercq on Occasion of His 80th Birthday

Nucleoside and nucleotide analogues are essential antivirals in the treatment of infectious diseases such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), hepatitis C virus (HCV), herpes simplex virus (HSV), varicella-zoster virus (VZV), and human cytomegalovirus (HCMV). To celebrate the 80th birthday of Prof. Dr. Erik De Clercq on 28 March 2021, this review provides an overview of his contributions to eight approved nucleos(t)ide drugs: (i) three adenosine nucleotide analogues, namely tenofovir disoproxil fumarate (Viread^®) and tenofovir alafenamide (Vemlidy^®) against HIV and HBV infections and adefovir dipivoxil (Hepsera^®) against HBV infections; (ii) two thymidine nucleoside analogues, namely brivudine (Zostex^®) against HSV-1 and VZV infections and stavudine (Zerit^®) against HIV infections; (iii) two guanosine analogues, namely valacyclovir (Valtrex^®, Zelitrex^®) against HSV and VZV and rabacfosadine (Tanovea^®-CA1) for the treatment of lymphoma in dogs; and (iv) one cytidine nucleotide analogue, namely cidofovir (Vistide^®) for the treatment of HCMV retinitis in AIDS patients. Although adefovir dipivoxil, stavudine, and cidofovir are virtually discontinued for clinical use, tenofovir disoproxil fumarate and tenofovir alafenamide remain the most important antivirals against HIV and HBV infections worldwide. Overall, the broad-spectrum antiviral potential of nucleos(t)ide analogues supports their development to treat or prevent current and emerging infectious diseases worldwide.

Overview of Biologically Active Nucleoside Phosphonates

The use of the phosphonate motif featuring a carbon-phosphorous bond as bioisosteric replacement of the labile P–O bond is widely recognized as an attractive structural concept in different areas of medicinal chemistry, since it addresses the very fundamental principles of enzymatic stability and minimized metabolic activation. This review discusses the most influential successes in drug design with special emphasis on nucleoside phosphonates and their prodrugs as antiviral and cancer treatment agents. A description of structurally related analogs able to interfere with the transmission of other infectious diseases caused by pathogens like bacteria and parasites will then follow. Finally, molecules acting as agonists/antagonists of P2X and P2Y receptors along with nucleotidase inhibitors will also be covered. This review aims to guide readers through the fundamentals of nucleoside phosphonate therapeutics in order to inspire the future design of molecules to target infections that are refractory to currently available therapeutic options.

A Structural Analysis of the FDA Green Book-Approved Veterinary Drugs and Roles in Human Medicine

The FDA Green Book is a list of all drug products that have been approved by the FDA for use in veterinary medicine. The Green Book, as published, lacks structural information corresponding to approved drugs. To address this gap, we have compiled the structural data for all FDA Green Book drugs approved through the end of 2019. Herein we discuss the relevance of this data set to human drugs in the context of structural classes and physicochemical properties. Analysis reveals that physicochemical properties are highly optimized and consistent with a high probability of favorable drug metabolism and pharmacokinetic properties, including good oral bioavailability for most compounds. We provide a detailed analysis of this data set organized on the basis of structure and function. Slightly over half (51%) of vet drugs are also approved in human medicine. Combination drugs are biologics are also discussed.

Tenofovir alafenamide does not inhibit mitochondrial function and cholesterol biosynthesis in human T lymphoblastoid cell line

In clinical trials, the concentration of tenofovir diphosphate (TFV-DP) in peripheral mononuclear cells was 4 to 5-fold higher in individuals treated with tenofovir alafenamide (TAF) compared to individuals treated with tenofovir disoproxil fumarate (TDF). We hypothesized that the higher intracellular accumulation of TFV-DP could cause mitochondrial toxicity from either polymerase gamma (Pol-γ)-dependent or Pol-γ-independent mechanism(s). To test this hypothesis, we cultured human T lymphoblastoid cell line (CEM cells) for up to 12 days with TAF or TDF (multiplicities of C_max) to investigate the effects on mitochondrial function and respiration, and cholesterol biosynthesis. Both TAF and TDF treatments had no significant effect on cell growth, mitochondrial potential (ΔΨ), production of reactive oxygen species (ROS), and mitochondrial respiratory parameters. TAF had no statistically significant effect on expression of Pol-γ mRNA, mitochondria DNA (mtDNA) content, expression of proteins of the electron transport chain (ETC), and key genes of cholesterol biosynthesis. TDF had significant reduction in mtDNA content at 8xC_max, and statistically significant reduction in mRNA expression of squalene epoxidase (SQLE). Our findings do not support our hypothesis that the higher intracellular accumulation of TFV-DP in cells treated with TAF could cause mitochondrial dysfunction. In conclusion, our findings add to the emerging data that TAF may have a low potential for causing mitochondrial toxicity in HIV-infected individuals on TAF-containing regimens.

Rabacfosadine for naïve canine intermediate to large cell lymphoma: Efficacy and adverse event profile across three prospective clinical trials

While current lymphoma therapies induce remission in most dogs, drug‐resistant relapse is common, creating a need for novel agents. Rabacfosadine (RAB), a double prodrug of the acyclic nucleotide phosphonate 9‐(2‐phosphonylmethoxyethel) guanine (PMEG), preferentially targets lymphoma cells with reduced systemic toxicity compared with PMEG. Previous studies evaluating RAB administered every 21 days have suggested efficacy in both naïve and relapsed subjects; however, no large studies of RAB as a single agent have been reported in previously untreated dogs with intermediate to large cell lymphoma. The purpose of this study was to evaluate the safety and efficacy of RAB in dogs with previously untreated (excluding corticosteroids) lymphoma. Sixty‐three dogs received up to five RAB treatments every 21 days (16 at 0.82 mg/kg and 47 at 1.0 mg/kg) as a 30 minutes intravenous infusion, with (n = 23) or without (n = 40) concurrent corticosteroids. Response assessment and adverse event (Ae) evaluation were performed every 21 days via Veterinary Cooperative Oncology Group (VCOG) criteria. The overall response rate was 87% (52% CR, 35% PR). The overall median progression free interval was 122 days (199 for CR, 89 for PR and 153 days for all responders). T‐cell immunophenotype and corticosteroid pre‐treatment were predictive of inferior outcomes on multivariate analysis. AEs were most commonly of gastrointestinal origin (hyporexia/diarrhoea) and generally resolved with supportive treatment and/or dosage adjustment. Three dogs experienced VCOG‐CTCAE grade 5 delayed pulmonary fibrosis. In conclusion, RAB administered every 3 weeks is generally well tolerated and demonstrates substantial antitumour activity in dogs with previously untreated intermediate to large cell lymphoma.

Predictive modeling for cancer drug discovery using canine models

INTRODUCTION

Rodent models of cancer lack many features associated with the disease in humans. Because dogs closely share an environment with humans, as well as comparable pathophysiology of cancer, they represent a powerful model with which to study novel approaches to cancer treatment.

AREAS COVERED

The authors summarize the weaknesses of rodent models of cancer and the ongoing need for better animal models with which to study potential therapeutic approaches. The homology of cancer in dogs and humans is described, along with examples specific to several common cancer types.

EXPERT OPINION

Laboratory mice and rats will continue to play a central role in cancer research; however, because of a variety of limitations, pet dogs with spontaneous cancer offer unique opportunities for research and should be included in the preclinical development of therapeutic compounds. Environmental homology between dogs and humans, along with biological and molecular similarities present circumstances that strengthen the translational rigor of studies conducted using canine patients. Progress will depend on a sufficient number of dogs to be diagnosed with cancer and available for use in studies; and essential to this will be the availability of enhanced resources for diagnosis of cancer in canine patients and reliable coordination between research scientists, veterinarians, and physicians.

Concurrent use of rabacfosadine and L-asparaginase for relapsed or refractory multicentric lymphoma in dogs

Background

Rabacfosadine (RAB), a novel antineoplastic agent conditionally licensed for the treatment of lymphoma in dogs, is efficacious in both naïve and previously treated dogs. Its use in combination with L‐asparaginase (L‐ASP) has not been studied.

Hypothesis/Objectives

To evaluate the safety and efficacy of L‐ASP given concurrently with RAB in dogs with relapsed multicentric lymphoma.

Animals

Fifty‐two dogs with relapse of lymphoma after treatment with at least 1 doxorubicin‐based chemotherapy protocol.

Methods

Open‐label, multicenter, prospective single‐arm clinical trial. Dogs were treated with RAB at 1.0 mg/kg IV every 21 days for up to a total of 5 doses. L‐asparaginase was administered at 400 IU/kg SQ concurrently with the first 2 treatments of RAB.

Results

The overall response rate (ORR) for all dogs was 67%, with 19 dogs (41%) achieving a complete response (CR). The median progression‐free survival time (MPFS) was 63 days (range 5‐428 days). Dogs experiencing a CR as their best response had an MPFS of 144 days (range 44‐428 days). Adverse events were similar to previous studies evaluating single agent RAB. Failure to achieve a CR and having previously received L‐ASP were negative prognostic factors on multivariate analysis.

Conclusions and Clinical Importance

Concurrent RAB/L‐ASP appears to be both efficacious and safe for treating relapsed multicentric lymphoma in dogs. Adverse events were most often mild and no unexpected toxicoses were observed.

Advanced Cancer Imaging Applied in the Comparative Setting

The potential for companion (pet) species with spontaneously arising tumors to act as surrogates for preclinical development of advanced cancer imaging technologies has become more apparent in the last decade. The utility of the companion model specifically centers around issues related to body size (including spatial target/normal anatomic characteristics), physical size and spatial distribution of metastasis, tumor heterogeneity, the presence of an intact syngeneic immune system and a syngeneic tumor microenvironment shaped by the natural evolution of the cancer. Companion species size allows the use of similar equipment, hardware setup, software, and scan protocols which provide the opportunity for standardization and harmonization of imaging operating procedures and quality assurance across imaging protocols, imaging hardware, and the imaged species. Murine models generally do not replicate the size and spatial distribution of human metastatic cancer and these factors strongly influence image resolution and dosimetry. The following review will discuss several aspects of comparative cancer imaging in more detail while providing several illustrative examples of investigational approaches performed or currently under exploration at our institutions. Topics addressed include a discussion on interested consortia; image quality assurance and harmonization; image-based biomarker development and validation; contrast agent and radionuclide tracer development; advanced imaging to assess and predict response to cytotoxic and immunomodulatory anticancer agents; imaging of the tumor microenvironment; development of novel theranostic approaches; cell trafficking assessment via non-invasive imaging; and intraoperative imaging to inform surgical oncology decision making. Taken in totality, these comparative opportunities predict that safety, diagnostic and efficacy data generated in companion species with naturally developing and progressing cancers would better recapitulate the human cancer condition than that of artificial models in small rodent systems and ultimately accelerate the integration of novel imaging technologies into clinical practice. It is our hope that the examples presented should serve to provide those involved in cancer investigations who are unfamiliar with available comparative methodologies an understanding of the potential utility of this approach.

Naturally occurring cancers in pet dogs as pre-clinical models for cancer immunotherapy

Despite the significant progress in tumor prevention, early detection, diagnosis and treatment made over recent decades, cancer is still an enormous public health challenge all around the world, with the number of people affected increasing every year. A great deal of effort is therefore being devoted to the search for novel safe, effective and economically sustainable treatments for the growing population of neoplastic patients. One main obstacle to this process is the extremely low percentage of therapeutic approaches that, after successfully passing pre-clinical testing, actually demonstrate activity when finally tested in humans. This disappointing and expensive failure rate is partly due to the pre-clinical murine models used for in vivo testing, which cannot faithfully recapitulate the multifaceted nature and evolution of human malignancies. These features are better mirrored in natural disease models, i.e., companion animals affected by cancers. Herein, we discuss the relevance of spontaneous canine tumors for the evaluation of the safety and anti-tumor activity of novel therapeutic strategies before in-human trials, and present our experience in the development of a vaccine that targets chondroitin sulphate proteoglycan (CSPG)4 as an example of these comparative oncology studies.

Clinical, Pathological, and Ethical Considerations for the Conduct of Clinical Trials in Dogs with Naturally Occurring Cancer: A Comparative Approach to Accelerate Translational Drug Development

The role of comparative oncology in translational research is receiving increasing attention from drug developers and the greater biomedical research community. Pet dogs with spontaneous cancer are important and underutilized translational models, owing to dogs' large size and relative outbreeding, combined with their high incidence of certain tumor histotypes with significant biological, genetic, and histological similarities to their human tumor counterparts. Dogs with spontaneous tumors naturally develop therapy resistance and spontaneous metastasis, all in the context of an intact immune system. These fundamental features of cancer biology are often lacking in induced or genetically engineered preclinical tumor models and likely contribute to their poor predictive value and the associated overall high failure rate in oncology drug development. Thus, the conduct of clinical trials in pet dogs with naturally occurring cancer represents a viable surrogate and valuable intermediary step that should be increasingly incorporated into the cancer drug discovery and development pipeline. The development of molecular-targeted therapies has resulted in an expanded role of the pathologist in human oncology trials, and similarly the expertise of veterinary pathologists will be increasingly valuable to all phases of comparative oncology trial design and conduct. In this review, we provide a framework of clinical, ethical, and pathology-focused considerations for the increasing integration of translational research investigations in dogs with spontaneous cancer as a means to accelerate clinical cancer discovery and drug development.

Direct and indirect quantification of phosphate metabolites of nucleoside analogs in biological samples

Nucleoside reverse transcriptase inhibitors (NRTIs) are prodrugs that require intracellular phosphorylation to active triphosphate nucleotide metabolites (NMs) for their pharmacological activity. However, monitoring these pharmacologically active NMs is challenging due to their instability, high hydrophilicity, and their low concentrations in blood and tissues. Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the gold standard technique for the quantification of NRTIs and their phosphorylated NMs. In this review, an overview of the publications describing the quantitative analysis of intracellular and total tissue concentration of NMs is presented. The focus of this review is the comparison of the different approaches and challenges associated with sample collection, tissue homogenization, cell lysis, cell counting, analyte extraction, sample storage conditions, and LC-MS analysis. Quantification methods of NMs via LC-MS can be categorized into direct and indirect methods. In the direct LC-MS methods, chromatographic retention of the NMs is accomplished by ion-exchange (IEX), ion-pairing (IP), hydrophilic interaction (HILIC), porous graphitic carbon (PGC) chromatography, or capillary electrophoresis (CE). In indirect methods, parent nucleosides are 1st generated from the dephosphorylation of NMs during sample preparation and are then quantified by reverse phase LC-MS as surrogates for their corresponding NMs. Both approaches have advantages and disadvantages associated with them, which are discussed in this review.

Xanthine-based acyclic nucleoside phosphonates with potent antiviral activity against varicella-zoster virus and human cytomegalovirus

While noncanonic xanthine nucleotides XMP/dXMP play an important role in balancing and maintaining intracellular purine nucleotide pool as well as in potential mutagenesis, surprisingly, acyclic nucleoside phosphonates bearing a xanthine nucleobase have not been studied so far for their antiviral properties. Herein, we report the synthesis of a series of xanthine-based acyclic nucleoside phosphonates and evaluation of their activity against a wide range of DNA and RNA viruses. Two acyclic nucleoside phosphonates within the series, namely 9-[2-(phosphonomethoxy)ethyl]xanthine (PMEX) and 9-[3-hydroxy-2-(phosphonomethoxy)propyl]xanthine (HPMPX), were shown to possess activity against several human herpesviruses. The most potent compound was PMEX, a xanthine analogue of adefovir (PMEA). PMEX exhibited a single digit µM activity against VZV (EC₅₀ = 2.6 µM, TK⁺ Oka strain) and HCMV (EC₅₀ = 8.5 µM, Davis strain), while its hexadecyloxypropyl monoester derivative was active against HSV-1 and HSV-2 (EC₅₀ values between 1.8 and 4.0 µM). In contrast to acyclovir, PMEX remained active against the TK^- VZV 07-1 strain with EC₅₀ = 4.58 µM. PMEX was suggested to act as an inhibitor of viral DNA polymerase and represents the first reported xanthine-based acyclic nucleoside phosphonate with potent antiviral properties.

Novel Treatments for Lymphoma

Lymphoma is a common disease in companion animals. Although conventional chemotherapy has the potential to induce remission and prolong life, relapse is common, and novel treatments are needed to improve outcome. This review discusses recent modifications/adjustments to conventional standard of care therapy for canine and feline lymphoma, as well as cutting-edge immunotherapy and small-molecule-based approaches that are in varying stages of regulatory approval.

Fifty Years in Search of Selective Antiviral Drugs

Fifty years of research (1968-2018) toward the identification of selective antiviral drugs have been primarily focused on antiviral compounds active against DNA viruses (HSV, VZV, CMV, HBV) and retroviruses (HIV). For the treatment of HSV infections the aminoacyl esters of acyclovir were designed, and valacyclovir became the successor of acyclovir in the treatment of HSV and VZV infections. BVDU (brivudin) still stands out as the most potent among the marketed compounds for the treatment of VZV infections (i.e., herpes zoster). In the treatment of HIV infections 10 tenofovir-based drug combinations have been marketed, and tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF) have also proved effective in the treatment of HBV infections. As a spin-off of our anti-HIV research, a CXCR4 antagonist AMD-3100 was found to be therapeutically useful as a stem cell mobilizer, and has since 10 years been approved for the treatment of some hematological malignancies.

Nucleobase Modified Adefovir (PMEA) Analogues as Potent and Selective Inhibitors of Adenylate Cyclases from Bordetella pertussis and Bacillus anthracis

A series of 13 acyclic nucleoside phosphonates (ANPs) as bisamidate prodrugs was prepared. Five compounds were found to be non-cytotoxic and selective inhibitors of Bordetella pertussis adenylate cyclase toxin (ACT) in J774A.1 macrophage cell-based assays. The 8-aza-7-deazapurine derivative of adefovir (PMEA) was found to be the most potent ACT inhibitor in the series (IC50 =16 nm) with substantial selectivity over mammalian adenylate cyclases (mACs). AC inhibitory properties of the most potent analogues were confirmed by direct evaluation of the corresponding phosphonodiphosphates in cell-free assays and were found to be potent inhibitors of both ACT and edema factor (EF) from Bacillus anthracis (IC50 values ranging from 0.5 to 21 nm). Moreover, 7-halo-7-deazapurine analogues of PMEA were discovered to be potent and selective mammalian AC1 inhibitors (no inhibition of AC2 and AC5) with IC50 values ranging from 4.1 to 5.6 μm in HEK293 cell-based assays.

Tanovea® for the treatment of lymphoma in dogs

Tanovea® (first named GS-9219, then VDC-1101, generic name: rabacfosadine) is a pro-prodrug or "double" prodrug of PMEG [9-(2-phosphonylmethoxyethyl)guanine], which has been conditionally approved by the US FDA (Food and Drug Administration) for the treatment of lymphoma in dogs. Tanovea has been demonstrated to be effective against non-Hodgkin's lymphoma (NHL) in dogs, as well as canine cutaneous T-cell lymphoma, spontaneous canine multiple myeloma, naïve canine multicentric lymphoma and relapsed canine B-cell lymphoma. As a double prodrug of PMEG, GS-9219 is first converted intracellularly by hydrolysis to cPr-PMEDAP, then deaminated to PMEG, which is then phosphorylated twice to its active metabolite PMEGpp, acting at the level of the cellular DNA polymerases.

A novel lipid prodrug strategy for sustained delivery of hexadecyloxypropyl 9-[2-(phosphonomethoxy)ethyl]guanine (HDP-PMEG) on unwanted ocular proliferation

Proliferative vitreoretinopathy (PVR) is a blinding eye disease and there is no effective pharmacological measure to prevent PVR development. The difficulty comes from lack of potent antiproliferative agent and lack of sustained delivery to cover high-risk time window for PVR to develop. Lipid prodrug of PMEG, hexadecyloxypropyl 9-[(2-phosphonomethoxy)ethyl]guanine (HDP-PMEG), was prepared and was evaluated as a pharmacological adjuvant to surgical management of PVR. A dose-escalation study determined that the highest nontoxic dose for intravitreal use in pigmented rabbits was 3 µg per eye. The genotoxicity of HDP-PMEG was harnessed as a perioperative preventative measure against PVR in a rabbit eye model while the sustained intravitreal pharmacological effect was evaluated on a laser-induced fibrovascular model in rat eye. After intravitreal 3 µg, HDP-PMEG particles in the rabbit vitreous was visible for at least 6 weeks. A single 50-min intravitreal infusion of HDP-PMEG demonstrated significant inhibition of PVR formation when compared with the eyes infused with only BSS (BSS vs. HDP-PMEG: estimate = 1.14, OR = 3.1, p = .027). A single intravitreal 104 ng (equivalent to 3 µg for rabbit eye) of HDP-PMEG significantly inhibit laser-induced fibrovascular proliferation in rat eye by 55% (least square mean pixel, BSS = 4763569.5 vs. HDP-PMEG = 2148129.7, p < .0001, generalized estimating equation [GEE]). Retinal fluorescein angiography showed the odds for BSS intervened eyes to have higher-rated FA leaking grades were 38.5 times compared with HDP-PMEG treated eyes (p < .0001, GEE). Our study results indicate that single intravitreal HDP-PMEG may be a promising ocular drug delivery as a perioperative intervention to prevent PVR reoccurrence following primary surgical management.

Alternating Rabacfosadine/Doxorubicin: Efficacy and Tolerability in Naïve Canine Multicentric Lymphoma

Background

Standard of care treatment for multicentric lymphoma in dogs remains doxorubicin (DOX)‐based combination chemotherapy, but owners may hesitate to commit the time and financial resources to complete such a protocol, typically requiring 12–16 visits. Rabacfosadine (RAB), a double prodrug of the nucleotide analog 9‐(2‐phosphonylmethoxyethyl) guanine, has substantial single‐agent activity in dogs with lymphoma, and a different mechanism of action than DOX.

Hypothesis/Objectives

Our objective was to evaluate the efficacy and adverse effect (AE) profile of alternating doses of RAB and DOX in dogs with naïve multicentric lymphoma.

Animals

Fifty‐four dogs with previously untreated lymphoma.

Methods

Open‐label, multicenter prospective clinical trial. Dogs received alternating RAB (1.0 mg/kg IV weeks 0, 6, 12) and DOX (30 mg/m² IV weeks 3, 9, 15). Dogs that achieved complete response (CR) were followed by monthly evaluations. Complete clinicopathological evaluation and assessment of remission and AEs were performed every 21 days.

Results

The overall response rate was 84% (68%; CR; 16%; partial response [PR)]. The overall median progression‐free interval (PFI) was 194 days (216 for CR and 63 for PR). Most AEs were mild and self‐limiting: gastrointestinal and hematologic AEs were most common. Thirteen dogs experienced dermatologic AEs, and 2 dogs developed grade 5 pulmonary fibrosis.

Conclusions and Clinical Importance

Alternating RAB/DOX generally was well tolerated and resulted in PFIs comparable to standard DOX‐based multi‐agent protocols, with fewer treatment visits. Most adverse events were mild or moderate and self‐limiting. Further studies are warranted to explore long‐term outcome and other RAB chemotherapy combinations.

Metabolism, Biochemical Actions, and Chemical Synthesis of Anticancer Nucleosides, Nucleotides, and Base Analogs

Nucleoside, nucleotide, and base analogs have been in the clinic for decades to treat both viral pathogens and neoplasms. More than 20% of patients on anticancer chemotherapy have been treated with one or more of these analogs. This review focuses on the chemical synthesis and biology of anticancer nucleoside, nucleotide, and base analogs that are FDA-approved and in clinical development since 2000. We highlight the cellular biology and clinical biology of analogs, drug resistance mechanisms, and compound specificity towards different cancer types. Furthermore, we explore analog syntheses as well as improved and scale-up syntheses. We conclude with a discussion on what might lie ahead for medicinal chemists, biologists, and physicians as they try to improve analog efficacy through prodrug strategies and drug combinations.

Design and Synthesis of Fluorescent Acyclic Nucleoside Phosphonates as Potent Inhibitors of Bacterial Adenylate Cyclases

Bordetella pertussis adenylate cyclase toxin (ACT) and Bacillus anthracis edema factor (EF) are key virulence factors with adenylate cyclase (AC) activity that substantially contribute to the pathogenesis of whooping cough and anthrax, respectively. There is an urgent need to develop potent and selective inhibitors of bacterial ACs with prospects for the development of potential antibacterial therapeutics and to study their molecular interactions with the target enzymes. Novel fluorescent 5-chloroanthraniloyl-substituted acyclic nucleoside phosphonates (Cl-ANT-ANPs) were designed and synthesized in the form of their diphosphates (Cl-ANT-ANPpp) as competitive ACT and EF inhibitors with sub-micromolar potency (IC50 values: 11-622 nm). Fluorescence experiments indicated that Cl-ANT-ANPpp analogues bind to the ACT active site, and docking studies suggested that the Cl-ANT group interacts with Phe306 and Leu60. Interestingly, the increase in direct fluorescence with Cl-ANT-ANPpp having an ester linker was strictly calmodulin (CaM)-dependent, whereas Cl-ANT-ANPpp analogues with an amide linker, upon binding to ACT, increased the fluorescence even in the absence of CaM. Such a dependence of binding on structural modification could be exploited in the future design of potent inhibitors of bacterial ACs. Furthermore, one Cl-ANT-ANP in the form of a bisamidate prodrug was able to inhibit B. pertussis ACT activity in macrophage cells with IC50 =12 μm.

Bisamidate Prodrugs of 2-Substituted 9-[2-(Phosphonomethoxy)ethyl]adenine (PMEA, adefovir) as Selective Inhibitors of Adenylate Cyclase Toxin from Bordetella pertussis

Novel small-molecule agents to treat Bordetella pertussis infections are highly desirable, as pertussis (whooping cough) remains a serious health threat worldwide. In this study, a series of 2-substituted derivatives of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA, adefovir), in their isopropyl ester bis(L-phenylalanine) prodrug form, were designed and synthesized as potent inhibitors of adenylate cyclase toxin (ACT) isolated from B. pertussis. The series consists of PMEA analogues bearing either a linear or branched aliphatic chain or a heteroatom at the C2 position of the purine moiety. Compounds with a small C2 substituent showed high potency against ACT without cytotoxic effects as well as good selectivity over human adenylate cyclase isoforms AC1, AC2, and AC5. The most potent ACT inhibitor was found to be the bisamidate prodrug of the 2-fluoro PMEA derivative (IC50 =0.145 μM). Although the bisamidate prodrugs reported herein exhibit overall lower activity than the bis(pivaloyloxymethyl) prodrug (adefovir dipivoxil), their toxicity and plasma stability profiles are superior. Furthermore, the bisamidate prodrug was shown to be more stable in plasma than in macrophage homogenate, indicating that the free phosphonate can be effectively distributed to target tissues, such as the lungs. Thus, ACT inhibitors based on acyclic nucleoside phosphonates may represent a new strategy to treat whooping cough.

Symmetrical diamidate prodrugs of nucleotide analogues for drug delivery

The use of pronucleotides to circumvent the well-known drawbacks of nucleotide analogs has played a significant role in the area of antiviral and anticancer drug delivery. Several motifs have been designed to mask the negative charges on the phosphorus moiety of either nucleoside monophosphates or nucleoside phosphonates, in order to increase their hydrophobicity and allow entry of the compound into the cell. Among them the bis-amidate analogs, having two identical amino acids as masking groups through a P-N bond, represent a more recent approach for the delivery of nucleotide analogs, endowed with antiviral or anticancer activity. Different synthetic strategies are commonly used for preparing phosphorodiamidates of nucleosides. In this protocol, we would like to focus on the description of the synthetic methodology that in our hand gave the best results using 2'-3'-didehydro-2'-3'-dideoxythymidine (d4T, Stavudine) as model nucleoside. A second strategy for preparing diamidates of nucleoside phosphonates will be reported using {[2-(6-amino-9 H-purin-9-yl)ethoxy]methyl}phosphonic acid (PMEA, adefovir) as model substrate.

Insights into the mechanism of action of cidofovir and other acyclic nucleoside phosphonates against polyoma- and papillomaviruses and non-viral induced neoplasia

Acyclic nucleoside phosphonates (ANPs) are well-known for their antiviral properties, three of them being approved for the treatment of human immunodeficiency virus infection (tenofovir), chronic hepatitis B (tenofovir and adefovir) or human cytomegalovirus retinitis (cidofovir). In addition, cidofovir is mostly used off-label for the treatment of infections caused by several DNA viruses other than cytomegalovirus, including papilloma- and polyomaviruses, which do not encode their own DNA polymerases. There is considerable interest in understanding why cidofovir is effective against these small DNA tumor viruses. Considering that papilloma- and polyomaviruses cause diseases associated either with productive infection (characterized by high production of infectious virus) or transformation (where only a limited number of viral proteins are expressed without synthesis of viral particles), it can be envisaged that cidofovir may act as antiviral and/or antiproliferative agent. The aim of this review is to discuss the advances in recent years in understanding the mode of action of ANPs as antiproliferative agents, given the fact that current data suggest that their use can be extended to the treatment of non-viral related malignancies.

Phase II evaluation of VDC-1101 in canine cutaneous T-cell lymphoma

BACKGROUND

Canine cutaneous T-cell lymphoma (CTCL) is an uncommon disease for which efficacious therapies are lacking. The novel anticancer nucleotide prodrug VDC-1101 (formerly known as GS-9219) has shown efficacy in dogs with multicentric lymphoma. One of the observed adverse effects with this drug was a skin change characterized by hair loss, erythema, and pruritus, implying delivery of VDC-1101 to the skin.

HYPOTHESIS/OBJECTIVES

The primary study objective was to identify the objective response rate (ORR) to VDC-1101 in canine CTCL; secondary objectives included characterization of progression-free survival (PFS) and adverse events (AEs).

ANIMALS

Twelve dogs with chemotherapy-naïve or relapsed, histologically and immunohistochemically confirmed CTCL.

METHODS

Dogs received VDC-1101 as a 30-minute IV infusion once every 21 days. Prednisone (1 mg/kg PO q48h) was administered concurrently.

RESULTS

In 11 evaluable patients, responses included 1 complete response (CR), 4 partial responses (PR), 2 stable disease (SD), and 4 progressive disease for an ORR of 45% and biologic response rate (CR/PR/SD) of 64%. The median PFS was 37.5 days (26 to >399 days), which includes 1 durable and ongoing CR (>1 year). Gastrointestinal and hematologic AEs were mild; no dogs developed grade 3 or 4 AEs. Three dogs developed dermatopathies and 1 of these dogs was removed from the study as a result of this AE.

CONCLUSIONS AND CLINICAL IMPORTANCE

VDC-1101 has activity against canine CTCL and could provide another treatment option in a disease process with a poor prognosis.

GS-9219/VDC-1101--a prodrug of the acyclic nucleotide PMEG has antitumor activity in spontaneous canine multiple myeloma

Background

Multiple myeloma (MM) is an important human and canine cancer for which novel therapies remain necessary. VDC-1101 (formerly GS-9219), a novel double prodrug of the anti-proliferative nucleotide analog 9-(2-phosphonylmethoxyethyl) guanine (PMEG), possesses potent cytotoxic activity in vitro in human lymphoblasts and leukemia cell lines and in vivo in spontaneous canine lymphoma. Given the similarity in lineage between lymphoma and MM, we hypothesized that VDC-1101 would be active against MM.

Results

We evaluated the in vitro antiproliferative effects of VDC-1101 against 3 human MM cell lines, and we performed a phase-II clinical trial in 14 dogs with spontaneous MM. Each dog was treated with a maximum of 6 doses of VDC-1101 monotherapy over 10–15 weeks. Dose-dependent antiproliferative activity was observed in all evaluated cell lines. Major antitumor responses (reduction of serum paraprotein and resolution of hypercalcemia, peripheral cytopenias and bone marrow plasmacytosis) were observed in 9 of 11 evaluable dogs for a median of 172 days, including a durable stringent complete response (>1047 days) in a dog with melphalan-refractory disease. 2 dogs were euthanized due to presumed pulmonary fibrosis; there were no other dose-limiting toxicities encountered.

Conclusions

In conclusion, VDC-1101 has significant anti-tumor activity at well-tolerated doses in spontaneous canine MM.

Dancing with chemical formulae of antivirals: a personal account

A chemical structure is a joy forever, and this is how I perceived the chemical structures of a number of antiviral compounds with which I have been personally acquainted over the past 3 decades: (1) amino acid esters of acyclovir (i.e. valaciclovir); (2) 5-substituted 2'-deoxyuridines (i.e. brivudin); (3) 2',3'-dideoxynucleoside analogues (i.e. stavudine); (4) acyclic nucleoside phosphonates (ANPs) (i.e. cidofovir, adefovir); (5) tenofovir disoproxil fumarate (TDF) and drug combinations therewith; (6) tenofovir alafenamide (TAF, GS-7340), a new phosphonoamidate prodrug of tenofovir; (7) pro-prodrugs of PMEG (i.e. GS-9191 and GS-9219); (8) new ANPs: O-DAPy and 5-aza-C phosphonates; (9) non-nucleoside reverse transcriptase inhibitors (NNRTIs): HEPT and TIBO derivatives; and (10) bicyclam derivatives (i.e. AMD3100).

Interactions with selected drug renal transporters and transporter-mediated cytotoxicity in antiviral agents from the group of acyclic nucleoside phosphonates

Members of acyclic nucleoside phosphonates (ANPs) possess antiviral and antiproliferative activities. However, several clinically important ANPs may cause renal injury, most likely due to their active accumulation in the renal tubular cells. The goal of this study was to investigate in vitro relationships between the affinity of several structurally related potent ANPs to selected human transporters and their cytotoxicity. SLC (solute carrier family) transporters (hOAT1, hOCT2, hCNT2, hCNT3) and ABC (ATP-binding cassette) transporters (MDR1, BCRP), which are typically expressed in the kidney, were included in the study. The transport and toxic parameters of the tested compounds were compared to those of two clinically approved ANPs, adefovir and tenofovir. Transport studies with transiently transfected cells were used as the main method in the experiments. Most of the ANPs studied showed the potency to interact with hOAT1. GS-9191, a double prodrug of PMEG, displayed an affinity for hOAT1 comparable with that of adefovir and tenofovir. No significant interaction of the tested ANPs with hOCT2, hCNT2 and hCNT3 was observed. Only GS-9191 was found to be a strong inhibitor for both MDR1 and BCRP. PMEO-DAPy showed the potency to interact with MDR1. Most of the tested substances caused a significant decrease in cellular viability in the cells transfected with hOAT1. Only with the exclusion of GS-9191, a relatively lipophilic compound, did the in vitro cytotoxicity of the ANPs closely correspond to their potential to interact with hOAT1. The increased cytotoxicity of the studied ANPs found in OAT1 transfected cells was effectively reduced by OAT inhibitors probenecid and quercetin. The higher cytotoxicity of the compounds with affinity to hOAT1 proved in the inhibitory experiments evidences that ANPs are not only inhibitors but also substrates of hOAT1. Any clear relationship between the potency of ANPs to inhibit the studied efflux transporters and their cytotoxicity was not demonstrated. In conclusion, the study documented that among the studied transporters hOAT1 seems to be the decisive determinant for renal handling in most of the tested ANPs. This transporter may also play an important role in the mechanism of their potential cytotoxic effects. These facts are in good accordance with previous findings in the clinically used ANPs.

Novel acyclic nucleotide analogues GS-343074 and GS-424044 demonstrate antiproliferative and pro-apoptotic activity in canine neoplastic cell lines

GS-9219, a novel prodrug of the nucleotide analogue 9-(2-phosphonylmethoxyethyl) guanine (PMEG) has significant activity as monotherapy in dogs with non-Hodgkin's lymphoma. Phase I trials have been initiated in humans based on the encouraging activity observed in canine lymphoma. Two new analogues of GS-9219 (GS-343074 and GS-424044) were recently produced for evaluation as potential novel antineoplastic agents against solid tumours. As a preclinical step, effect of GS-343074 and GS-424044 were evaluated against ten canine cancer cell lines for antiproliferative effect. Both analogues displayed antiproliferative activity against multiple canine cancer cell lines, although GS-343074 was more potent and of broader spectrum compared to GS-424044. Flow cytometric analysis of cells that experienced growth inhibition support apoptotic death as a mechanism of action for both analogues. On the basis of in vitro results described here, GS-343074 and GS-424044 show promise as novel anticancer agents in canine cancer.

The Holý Trinity: the acyclic nucleoside phosphonates

The Holý Trinity was named after Dr Antonín Holý (the Holý Trinity being an Unesco recognized monument in Olomouc, Czech Republic), who together with Dr John C. Martin (Gilead Sciences) and myself pioneered a new class of antiviral agents, the acyclic nucleoside phosphonates. These compounds have revolutionized the antiviral drug field with several drugs that have been approved for the treatment of various DNA virus infections (cidofovir), hepatitis B (adefovir), and AIDS (HIV infection; tenofovir). The latter is also available as its oral prodrug, tenofovir disoproxil fumarate, for the treatment of hepatitis B and in combination with emtricitabine ((-)FTC) for the treatment and prophylaxis of HIV infections and in combination with (-)FTC and other HIV inhibitors, that is, efavirenz, rilpivirine, or elvitegravir (and a pharmacoenhancer thereof, cobicistat), for the treatment of AIDS.

A novel type of acyclic nucleoside phosphonates derived from 2-(phosphonomethoxy)propanoic acid

A convenient and efficient synthesis of a novel class of acyclic nucleoside phosphonates derived from 2-(phosphonomethoxy)propanoic acid has been developed. The key step of the synthesis is the optimized oxidation of the 3-hydroxy-2-(phosphonomethoxy)propyl (HPMP) analogues to the corresponding 2'-carboxy-PME (CPME) derivatives using the TEMPO/NaClO2/NaClO oxidizing system. Although (S)-3-(adenin-9-yl)-2-(phosphonomethoxy)propanoic acid ((S)-CPMEA) has been designed as a compound with potential anti-HIV activity, none of the newly prepared CPME analogues exhibited any antiviral activity.

Adenosine deaminase-like protein 1 (ADAL1): characterization and substrate specificity in the hydrolysis of N(6)- or O(6)-substituted purine or 2-aminopurine nucleoside monophosphates

Human N(6)-methyl-AMP/dAMP aminohydrolase has been shown to be involved in metabolism of pharmacologically important N(6)-substituted purine nucleosides and 5'-monophosphate prodrugs thereof. This enzyme was cloned and expressed in E. coli, and mass spectroscopic analysis followed by amino acid sequence analyses indicated that the protein was adenosine deaminase-like protein isoform 1 (ADAL1). An extensive structure-activity relationship study showed that ADAL1 was able to catalyze removal of different alkyl groups not only from N(6)-substituted purine or 2-aminopurine nucleoside monophosphates but also from O(6)-substituted compounds. The ADAL1 activity was susceptible to modifications in the phosphate moiety but not to changes in the sugar moiety. Overall, our data indicated that ADAL1 specifically acts at the 6-position of purine and 2-aminopurine nucleoside monophosphates. Our results may help designing of new therapeutic nucleoside/nucleotide prodrugs with desired metabolic profiles. Furthermore, amino acid sequence analysis in conjunction with crystallographic data and metal analysis suggested that ADAL1 contains a catalytic zinc ion. Finally, a potential physiological role of ADAL1 is discussed.

Acyclic nucleoside phosphonates: An unfinished story

While cidofovir, adefovir and tenofovir are the three acyclic nucleoside phosphonates (ANPs) that have been licensed for clinical use (the latter as a single-, double- and triple-drug combination), there are many more ANPs that await their application for medical or veterinary use: (S)-HPMPA, (S)-HPMPDAP, cPrPMEDAP, (R)-HPMPO-DAPy, PMEO-DAPy, 5-X-PMEO-DAPy, (R)-PMPO-DAPy, (S)-HPMP-5-azaC, and cyclic (S)-HPMP-5-azaC, and alkoxyalkyl prodrugs thereof.