Synthesis and Evaluation of Novel Tetrahydronaphthyridine CXCR4 Antagonists with Improved Drug-like Profiles

Our first-generation CXCR4 antagonist TIQ15 was rationally modified to improve drug-like properties. Introducing a nitrogen atom into the aromatic portion of the tetrahydroisoquinoline ring led to several heterocyclic variants including the 5,6,7,8-tetrahydro-1,6-naphthyridine series, greatly reducing the inhibition of the CYP 2D6 enzyme. Compound 12a demonstrated the best overall properties after profiling a series of isomeric tetrahydronaphthyridine analogues in a battery of biochemical assays including CXCR4 antagonism, CYP 2D6 inhibition, metabolic stability, and permeability. The butyl amine side chain of 12a was substituted with various lipophilic groups to improve the permeability. These efforts culminated in the discovery of compound 30 as a potent CXCR4 antagonist (IC50 = 24 nM) with diminished CYP 2D6 activity, improved PAMPA permeability (309 nm/s), potent inhibition of human immunodeficiency virus entry (IC50 = 7 nM), a cleaner off-target in vitro safety profile, lower human ether a-go-go-related gene channel activity, and higher oral bioavailability in mice (% FPO = 27) compared to AMD11070 and TIQ15.

Amino-Heterocycle Tetrahydroisoquinoline CXCR4 Antagonists with Improved ADME Profiles via Late-Stage Buchwald Couplings

This work surveys a variety of diamino-heterocycles as an isosteric replacement for the piperazine substructure of our previously disclosed piperarinyl-tetrahydroisoquinoline containing CXCR4 antagonists. A late-stage Buchwald coupling route was developed for rapid access to final compounds from commercial building blocks. Among 13 analogs in this study, compound 31 embodying an aza-piperazine linkage was found to have the best overall profile with potent CXCR4 inhibitory activity and favorable in vitro absorption, distribution, metabolism, and excretion (ADME) properties. An analysis of the calculated physiochemical parameters (ROF, cLogD) and the experimental ADME attributes of the analogs lead to the selection of 31 for pharmacokinetic studies in mice. Compared with the clinical compound AMD11070, compound 31 has no CYP450 3A4 or 2D6 inhibition, higher metabolic stability and PAMPA permeability, greatly improved physiochemical parameters, and superior oral bioavailability (%F = 24). A binding rationale for 31 within CXCR4 was elucidated from docking and molecular simulation studies.

Quantitative Proteomic Analysis of Diabetes Mellitus in Heart Failure With Preserved Ejection Fraction

Diabetes mellitus (DM) is associated with a higher risk of heart failure hospitalization and mortality in patients with heart failure with preserved ejection fraction (HFpEF). Using SomaScan assays and proteomics analysis of plasma from participants in the TOPCAT (Treatment of Preserved Cardiac Function Heart Failure with an Aldosterone Antagonist) trial and the Penn Heart Failure Study, this study identified 10 proteins with significantly different expression in patients with HFpEF and DM. Of these, apolipoprotein M was found to mediate 72% (95% CI: 36% to 100%; p < 0.001) of the association between DM and the risk of cardiovascular death, aborted cardiac arrest, and heart failure hospitalization.

Effect of Serum Albumin Levels in Patients With Heart Failure With Preserved Ejection Fraction (from the TOPCAT Trial)

Little data are available regarding the determinants and prognostic significance of serum albumin in Heart Failure with Preserved Ejection Fraction (HFpEF). We sought to examine the phenotypic correlates of albumin and its independent prognostic implications in HFpEF. We analyzed data from 3,254 subjects enrolled the TOPCAT trial. We stratified subjects according to tertiles of albumin and examined differences in various phenotypic traits between these strata, including 8 protein biomarkers selected ad hoc and measured from frozen samples available in a subset of participants (n = 372). We also assessed the relationship between albumin and the trial primary endpoint. Lower albumin was associated with older age, black race, and greater prevalence of NYHA class III-IV, peripheral arterial disease, atrial fibrillation and diabetes mellitus. Lower albumin was also associated with increased levels of several inflammatory biomarkers, markers of liver fibrosis, albuminuria, and greater arterial stiffness, diastolic dysfunction and pulmonary hypertension. Albumin was a strong predictor of the primary trial endpoint, even after adjustment for the MAGGIC risk score (hazard ratio [HR] 0.72, confidence interval [CI] 0.67 to 0.78; p <0.0001) and prespecified traditional risk factors (HR 0.78, CI 0.71 to 0.85; p <0.0001). Lower albumin was strongly associated with a worse prognosis even well within normal ranges (>3.5 g/dL), with a sharp increase in risk between 4.6 and 3.6 g/dL. In conclusion, albumin is an integrated marker of various adverse processes in HFpEF, including inflammation, subclinical liver disease, arterial stiffness, and renal disease. Albumin is a powerful risk predictor independent of traditional risk prediction models, even within normal ranges.

Discovery of N-Alkyl Piperazine Side Chain Based CXCR4 Antagonists with Improved Drug-like Properties

A novel series of CXCR4 antagonists with piperidinyl and piperazinyl alkylamine side chains designed as butyl amine replacements are described. Several of these compounds showed similar activity to the parent compound TIQ-15 (5) in a SDF-1 induced calcium flux assay. Preliminary structure-activity relationship investigations led us to identify a series containing N-propyl piperazine side chain analogs exemplified by 16 with improved off-target effects as measured in a muscarinic acetylcholine receptor (mAChR) calcium flux assay and in a limited drug safety panel screen. Further efforts to explore SAR and optimize drug properties led to the identification of the N'-ethyl-N-propyl-piperazine tetrahydroisoquinoline derivative 44 and the N-propyl-piperazine benzimidazole compound 37, which gave the best overall profiles with no mAChR or CYP450 inhibition, good permeability in PAMPA assays, and metabolic stability in human liver microsomes.

Synthesis of Novel Tetrahydroisoquinoline CXCR4 Antagonists with Rigidified Side-Chains

A structure-activity relationship study of potent TIQ15-derived CXCR4 antagonists is reported. In this investigation, the TIQ15 side-chain was constrained to improve its drug properties. The cyclohexylamino congener 15a was found to be a potent CXCR4 inhibitor (IC₅₀ = 33 nM in CXCL12-mediated Ca²⁺ flux) with enhanced stability in liver microsomes and reduced inhibition of CYP450 (2D6). The improved CXCR4 antagonist 15a has potential therapeutic application as a single agent or combinatory anticancer therapy.

Discovery of Tetrahydroisoquinoline-Containing CXCR4 Antagonists with Improved in Vitro ADMET Properties

CXCR4 is a seven-transmembrane receptor expressed by hematopoietic stem cells and progeny, as well as by ≥48 different cancers types. CXCL12, the only chemokine ligand of CXCR4, is secreted within the tumor microenvironment, providing sanctuary for CXCR4⁺ tumor cells from immune surveillance and chemotherapeutic elimination by (1) stimulating prosurvival signaling and (2) recruiting CXCR4⁺ immunosuppressive leukocytes. Additionally, distant CXCL12-rich niches attract and support CXCR4⁺ metastatic growths. Accordingly, CXCR4 antagonists can potentially obstruct CXCR4-mediated prosurvival signaling, recondition the CXCR4⁺ leukocyte infiltrate from immunosuppressive to immunoreactive, and inhibit CXCR4⁺ cancer cell metastasis. Current small molecule CXCR4 antagonists suffer from poor oral bioavailability and off-target liabilities. Herein, we report a series of novel tetrahydroisoquinoline-containing CXCR4 antagonists designed to improve intestinal absorption and off-target profiles. Structure-activity relationships regarding CXCR4 potency, intestinal permeability, metabolic stability, and cytochrome P450 inhibition are presented.

Synthesis and SAR of 1,2,3,4-Tetrahydroisoquinoline-Based CXCR4 Antagonists

CXCR4 is the most common chemokine receptor expressed on the surface of many cancer cell types. In comparison to normal cells, cancer cells overexpress CXCR4, which correlates with cancer cell metastasis, angiogenesis, and tumor growth. CXCR4 antagonists can potentially diminish the viability of cancer cells by interfering with CXCL12-mediated pro-survival signaling and by inhibiting chemotaxis. Herein, we describe a series of CXCR4 antagonists that are derived from (S)-5,6,7,8-tetrahydroquinolin-8-amine that has prevailed in the literature. This series removes the rigidity and chirality of the tetrahydroquinoline providing 2-(aminomethyl)pyridine analogs, which are more readily accessible and exhibit improved liver microsomal stability. The medicinal chemistry strategy and biological properties are described.

The NF-kappa B signaling pathway is not required for Fas ligand gene induction but mediates protection from activation-induced cell death

Stimulation of T cells by antigens or mitogens triggers multiple signaling pathways leading to activation of genes encoding interleukin-2 and other growth-regulatory cytokines. The same stimuli also activate the gene encoding an apoptosis-inducing molecule, Fas ligand (FasL), which contributes to activation-induced cell death. It has been proposed that the signaling pathways involved in cytokine gene induction also contribute to activation-induced FasL expression; however, genetic evidence for this proposal is lacking. In the present study, the role of the NF-kappaB signaling pathway in FasL gene expression was examined using a mutant T cell line deficient in an essential NF-kappaB signaling component, IkappaB kinase gamma. These mutant cells have a blockade in signal-induced activation of NF-kappaB but remained normal in the activation of NF-AT and AP-1 transcription factors. Interestingly, the NF-kappaB signaling defect has no effect on mitogen-stimulated FasL gene expression, although it completely blocks the interleukin-2 gene induction. We further demonstrate that NF-kappaB activation is required for protecting T cells from apoptosis induction by mitogens and an agonistic anti-Fas antibody. These genetic results suggest that the NF-kappaB signaling pathway is not required for activation-induced FasL expression but rather mediates cell growth and protection from activation-induced cell death.

Extracellular catalytic subunit activity of the cAMP-dependent protein kinase in prostate cancer

The role of cAMP in cell growth and differentiation, gene expression, and neuronal function is mediated by the cAMP-dependent protein kinase (PKA). Differential expression of type I and type II PKA has been correlated with neoplastic transformation and differentiation, respectively. PKA is primarily an intracellular enzyme. However, it has been demonstrated that PKA may be associated with the plasma membrane and is exposed to the extracellular environment. Here we report the first evidence for the presence of a free extracellular kinase activity of PKA in the growth media of cultured prostate and other cancer cells, as well as in plasma samples from prostate cancer patients. This PKA activity is specific due to its phosphorylation of the PKA-specific substrate kemptide and its inhibition by the potent and specific PKA inhibitor PKI, but not by other protein kinase-inhibitory peptides. Intriguingly, this exoprotein kinase activity is cAMP independent, suggesting that only the catalytic subunit is secreted, and therefore the kinase activity is not modulated by the regulatory subunit of PKA. Western blot analysis of the culture supernatant from prostate cancer cells indicates the presence of the catalytic subunit. This increase in extracellular PKA catalytic subunit activity in prostate cancer may have profound effects on the tumorigenesis of prostate cancer and may serve as a novel marker and therapeutic target for the disease.

Somatic mutagenesis studies of NF-kappa B signaling in human T cells: evidence for an essential role of IKK gamma in NF-kappa B activation by T-cell costimulatory signals and HTLV-I Tax protein

NF-kappa B plays a pivotal role in normal T-cell activation and may also mediate human T-cell leukemia virus (HTLV)-induced T-cell transformation. Activation of NF-kappa B by both T-cell costimulatory signals and the HTLV Tax protein involves stimulation of I kappa B kinase (IKK). As a genetic approach to dissect the intermediate steps involved in NF-kappa B activation in human T cells, we performed somatic cell mutagenesis to isolate signaling-defective mutant Jurkat T-cell lines. One of the mutant cell lines was shown to have a specific blockade in the IKK signaling pathway but remained competent in the c-Jun N-terminal kinase and MAP kinase pathways. Interestingly, this mutant cell line lacks expression of IKK gamma, a non-catalytic component of the IKK complex. Expression of exogenous IKK gamma in the mutant cells restored NF-kappa B activation by both the T-cell costimulation agents and Tax. These findings provide genetic evidence for the requirement of IKK gamma in NF-kappa B signaling triggered by both T-cell costimulatory signals and HTLV-I Tax protein.

Telomere shortening by cisplatin in yeast nucleotide excision repair mutant

Telomeres are unique DNA tandem repeats that form the ends of eukaryotic chromosomes to protect the chromosomes from degradation and illegitimate recombination. In yeast, loss of telomere may be compensated for through the acquisition of new telomere by RAD52-mediated or RAD52-independent recombinational repair. In this report, the effects of cis-dichlorodiammine-platinum (II) (cisplatin) on telomere length and the role of nucleotide excision repair in telomere maintenance were examined in the yeast Saccharomyces cerevisiae. We showed that the SSL2 (RAD25) DNA repair yeast mutant exhibited a gradual shortening of the telomere in the presence of cisplatin. Further telomere shortening was prevented upon the withdrawal of cisplatin. Complementation of the mutant with the wild-type SSL2 (RAD25) gene abolished the cisplatin-induced telomere degradation. These results suggest that telomeres are susceptible to cisplatin-induced intrastrand crosslinks and that Ssl2 (Rad25) or the nucleotide excision repair pathway may play a critical role in the repair and the maintenance of telomere integrity.

The requirement of yeast Ssl2 (Rad25) for the repair of cisplatin-damaged DNA

Cisplatin is one of the most widely used anticancer agents. Cisplatin-induced cytotoxicity results from its ability to form cisplatin-DNA adducts within the cellular genome which can inhibit the transcription of genes and the replication of DNA. Cisplatin-adducts are primarily removed by the nucleotide excision repair (NER) pathway. The SSL2 (RAD25) gene of Saccharomyces cerevisiae, a homolog of the XPB (ERCC3) gene in humans, is involved in the nucleotide excision repair of UV-damaged DNA and is also required for cell viability. However, the role of Ssl2 (Rad25) in cisplatin sensitivity has not been examined. In this study, we have demonstrated that a yeast strain carrying the mutant allele SSL2-XP, a truncated form of SSL2 (RAD25) at the carboxyl terminus to mimic the human XPB (ERCC3) mutation, has increased cellular sensitivity to cisplatin in comparison to wild type cells. Analysis by host cell reactivation (HCR) assay further shows that Ssl2 (Rad25) is required for the repair of cisplatin-damaged DNA.

Cisplatin sensitivity in cAMP-dependent protein kinase mutants of Saccharomyces cerevisiae

The emergence of cisplatin resistance poses a significant problem to the treatment of a variety of human malignancies. Therefore, understanding the molecular basis of cisplatin resistance could improve the clinical effectiveness of this anticancer agent. Recently, our laboratory has demonstrated that cAMP-dependent protein kinase (PKA) mutants of the Chinese hamster ovary (CHO) and the mouse adrenocortical carcinoma Y1 cells exhibited increased resistance to cisplatin as well as other DNA-damaging drugs. Further studies showed that either the functional inactivation of PKA or the mutation in the regulatory subunit gene may cause increased recognition of cisplatin-damaged DNA and enhanced DNA repair capacity. In this study, we evaluated the role of PKA in modulating cellular sensitivity to cisplatin in a series of PKA mutants of Saccharomyces cerevisiae. Mutants with decreased kinase activity resulting from a srv2 mutation showed no alterations in cisplatin sensitivity. Complementation of TPK1 in a yeast strain containing mutant tpk1 and also tpk2 and tpk3 deletions did not significantly alter its sensitivity to this DNA-damaging agent. Yeast transformants containing increased kinase activity resulting from overexpression of RAS2Val19 or TPK1 and yeast strains having increased kinase activities due to mutations in the BCY1 gene also did not show alterations in their sensitivity to cisplatin. Therefore, results from these studies unambiguously demonstrate that changes in PKA activity have no effect on cisplatin sensitivity in Saccharomyces cerevisiae.

Effects of RIalpha overexpression on cisplatin sensitivity in human ovarian carcinoma cells

Our laboratory has found that Chinese hamster ovary (CHO) and mouse Y1 adrenocortical carcinoma PKA mutants with a defective R subunit, but not altered C subunits, exhibit increased resistance to cisplatin as well as other DNA-damaging agents. The mechanism of resistance may be associated with increased recognition of the cisplatin-damaged DNA and protein binding to the DNA lesion, thus enhancing DNA repair in the RI alpha mutants. These data suggest that mutation of RI alpha may confer resistance to cisplatin by affecting DNA repair activity. In the present study, we overexpressed RI alpha in human ovarian carcinoma A2780 cells to demonstrate that RI alpha can modulate cellular sensitivity to cisplatin. Retroviral-infected A2780 cells overexpressing wild-type RI alpha cDNA displayed a four- to eightfold greater sensitivity to cisplatin compared with parental cells. Overexpression of RI alpha in the CP70 cisplatin-resistant derivative of A2780 also increased the sensitivity of these cells to cisplatin. Therefore, enhanced expression of the RI alpha subunit of PKA sensitizes cells to the cytotoxic effects of this DNA-damaging agent. These data suggest that RI alpha may act directly, independent of the C subunit, to influence cellular sensitivity to cisplatin. Therefore, modulation of RI alpha expression or its functional status by pharmacological agents may potentially reverse cisplatin resistance in tumors.

Cisplatin resistance in cyclic AMP-dependent protein kinase mutants

The emergence of cisplatin resistance poses a major problem to the successful treatment of a variety of human malignancies. Therefore, understanding the molecular mechanisms that underlie cisplatin resistance could significantly improve the clinical efficacy of this cytotoxic agent. Various studies have described that cellular sensitivity to cisplatin can be influenced by several signal transduction pathways. In this review, we examine the role of the cyclic AMP-dependent protein kinase (PKA) in the modulation of drug resistance in cancer. By a somatic mutant genetic approach, the role of PKA in the development of resistance to chemotherapeutic agents has been investigated. A series of mutants with decreased PKA activity was examined for their sensitivity to cisplatin. PKA mutants with defective regulatory (RIalpha) subunits, but not altered catalytic (C) subunits, exhibit increased resistance to cisplatin, as well as other DNA-damaging agents. Furthermore, since RIalpha subunit mutants show enhanced DNA repair we, therefore, hypothesize that functional inactivation of PKA may result in increased recognition and repair of cisplatin lesions. Alternatively, it seems likely that mutation of the RIalpha subunit may affect cellular sensitivity to various anticancer drugs, suggesting that the RIalpha subunit may have other physiological functions in addition to inhibiting the kinase activity of the C subunit. Therefore, exploitation of cyclic AMP levels or functional alteration of the R subunit may potentiate the cytotoxicity of chemotherapeutic agents and circumvent drug resistance in cancer. More importantly, the altered pattern and mechanism of drug resistance may offer the opportunity to investigate novel regulatory functions of the RIalpha subunit of PKA.

Regulation of P-glycoprotein expression in cyclic AMP-dependent protein kinase mutants

Multidrug resistance (MDR) in cancer poses a major obstacle to the success of chemotherapy. We previously reported that cyclic AMP (cAMP)-resistant mutants of the Chinese hamster ovary and the mouse adrenal cortical carcinoma cells harboring defective regulatory (RI alpha) subunits of the cAMP-dependent protein kinase (PKA) are more sensitive than wild-type cells to chemotherapeutic agents that are substrates for P-glycoprotein. In addition, a transfectant overexpressing a mutant RI alpha cDNA showed similar increased sensitivity to these drugs. The altered drug sensitivity in the RI alpha mutants results from reduced expression of the mdr gene, suggesting that PKA may regulate its expression. In this study, we evaluated the sensitivity of several Chinese hamster ovary catalytic (C) subunit mutants to various anticancer drugs. Like the RI alpha subunit mutant, the C subunit mutants also exhibit decreased kinase activity and unresponsiveness to growth inhibition by cAMP. However, in contrast to the RI alpha subunit mutant, the C subunit mutants are not multidrug sensitive and maintain P-glycoprotein expression levels comparable to those of wild-type cells. Furthermore, the C subunit mutants display the same resistance patterns as wild-type cells to P-glycoprotein substrates, including Adriamycin, Taxol, and colchicine. No significant difference was observed in their sensitivity to non-MDR drugs, such as 5-fluorodeoxyuridine, between wild-type, RI alpha, and C subunit mutant cells. These results suggest that the increased multidrug sensitivity in the PKA mutant cells results from alteration of the RI alpha subunit and not the kinase activity, thus implying novel functions for the RI alpha subunit. Therefore, genetic alteration of the RI alpha subunit of PKA may modulate drug resistance in cancer.

Characterization of a cAMP-dependent protein kinase mutant resistant to cisplatin

The signal transduction pathway of cAMP, mediated by the cAMP-dependent protein kinase (PKA), is involved in the regulation of metabolisms, cell growth and differentiation and gene expression. Isolated PKA mutants from Chinese hamster ovary (CHO) cells were used in our laboratory to study the role of cAMP in the development of drug resistance in cancer. We have found that PKA mutants harboring a defective regulatory (RI alpha) subunit, but not the catalytic (C) subunit, are more resistant to the chemotherapeutic drug cisplatin. To clarify the role of PKA in cisplatin resistance, we have performed a step-wise selection with a CHO RI alpha subunit mutant cell line, 10248, for further resistance to cisplatin. A representative clone (10248/CDDP(R)-5) was used for further characterization. These cisplatin-resistant PKA mutant cells remained refractory to cAMP-induced growth inhibition and had decreased PKA activity comparable to the parental 10248 mutant cells. Furthermore, 10248/CDDP(R)-5 also exhibited cross-resistance to the nitrogen mustard melphalan but maintained the same sensitivity as wild-type cells to non-DNA-damaging agents such as methotrexate. The mechanism of resistance may be due to increased DNA repair as assessed by the host cell reactivation assay. We speculate that mutation and functional inactivation of PKA may result in deregulated growth response to cAMP, as well as the acquisition of resistance to cisplatin and other DNA-damaging agents in cancer.

Cisplatin resistance and regulation of DNA repair in cAMP-dependent protein kinase mutants

Drug resistance in cancer poses a major problem to the success of chemotherapy. Increased resistance to the DNA-damaging chemotherapeutic drug cisplatin may be associated with a variety of factors including decreased drug accumulation, increased intracellular levels of thiols, and increased DNA repair. We have found that mutants of the Chinese hamster ovary (CHO) and the mouse adrenocortical carcinoma Y1 cells harboring a defective regulatory subunit (RI) of the cAMP-dependent protein kinase (PKA) exhibited increased resistance to cisplatin. These mutants are cross-resistant to other DNA-damaging chemotherapeutic agents, including bleomycin and melphalan. In addition, wild-type CHO cells transfected with and overexpressing the yeast phosphodiesterase gene or a dominant mutant Rl alpha subunit gene also displayed similar increased resistance to cisplatin. However, mutants with altered catalytic (C) subunits showed a sensitivity to cisplatin similar to the wild-type cells. Further analysis by gel shift assay using cisplatin-damaged DNA as probes and nuclear extracts derived from the Rl subunit mutants showed increased binding of nuclear factor(s) to the damaged DNA. In addition, a host cell reactivation assay of DNA repair, using a cisplatin-damaged reporter plasmid, detected enhanced capacity for repair of DNA lesions in the PKA mutants. These results suggest that DNA repair may be increased in the PKA mutants. We speculate that functional inactivation of PKA may result in increased DNA repair and the acquisition of resistance to DNA-damaging anticancer drugs in cancer.