FIELD OF THE INVENTION
 This invention relates to compounds useful as prodrugs in the modulation of IL-12, IL-23 and/or IFNα by acting on Tyk-2 to cause signal transduction inhibition. Provided herein are prodrugs of amide-substituted heterocyclic compounds, compositions comprising such compounds, and methods of their use. The invention further pertains to pharmaceutical compositions containing at least one compound according to the invention that are useful for the treatment of conditions related to the modulation of IL-12, IL-23 and/or IFNα in a mammal.
BACKGROUND OF THE INVENTION
 The heterodimeric cytokines interleukin (IL)-12 and IL-23, which share a common p40 subunit, are produced by activated antigen-presenting cells and are critical in the differentiation and proliferation of Th1 and Th17 cells, two effector T cell lineages which play key roles in autoimmunity. IL-23 is composed of the p40 subunit along with a unique p19 subunit. IL-23, acting through a heterodimeric receptor composed of IL-23R and IL-12Rβ1, is essential for the survival and expansion of Th17 cells which produce pro-inflammatory cytokines such as IL-17A, IL-17F, IL-6 and TNF-α (McGeachy, M.J. et al., “The link between IL-23 and Th17 cell-mediated immune pathologies”, Semin. Immunol., 19:372-376 (2007)). These cytokines are critical in mediating the pathobiology of a number of autoimmune diseases, including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, and lupus. IL-12, in addition to the p40 subunit in common with IL-23, contains a p35 subunit and acts through a heterodimeric receptor composed of IL-12Rβ1 and IL-12Rβ2. IL-12 is essential for Th1 cell development and secretion of IFNγ, a cytokine which plays a critical role in immunity by stimulating MHC expression, class switching of B cells to IgG subclasses, and the activation of macrophages (Gracie, J.A. et al., “Interleukin-12 induces interferon-gamma-dependent switching of IgG alloantibody subclass”, Eur. J. Immunol., 26:1217-1221 (1996); Schroder, K. et al., “Interferon-gamma: an overview of signals, mechanisms and functions”, J. Leukoc. Biol., 75(2):163-189 (2004)).
 The importance of the p40-containing cytokines in autoimmunity is demonstrated by the discovery that mice deficient in either p40, p19, or IL-23R are protected from disease in models of multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, lupus and psoriasis, among others (Kyttaris, V.C. et al., “Cutting edge: IL-23 receptor deficiency prevents the development of lupus nephritis in C57BL/6-lpr/lpr mice”, J. Immunol., 184:4605-4609 (2010); Hong, K. et al., “IL-12, independently of IFN-gamma, plays a crucial role in the pathogenesis of a murine psoriasis like skin disorder”, J. Immunol., 162:7480-7491 (1999); Hue, S. et al., “Interleukin-23 drives innate and T cell-mediated intestinal inflammation”, J. Exp. Med., 203:2473-2483 (2006); Cua, D.J. et al., “Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain”, Nature, 421:744-748 (2003); Murphy, C.A. et al., “Divergent pro- and anti-inflammatory roles for IL-23 and IL-12 in joint autoimmune inflammation”, J. Exp. Med., 198:1951-1957 (2003)).
 In human disease, high expression of p40 and p19 has been measured in psoriatic lesions, and Th17 cells have been identified in active lesions in the brain from MS patients and in the gut mucosa of patients with active Crohn’s disease (Lee, E. et al., “Increased expression of interleukin 23 p19 and p40 in lesional skin of patients with psoriasis vulgaris”, J. Exp. Med., 199: 125-130 (2004); Tzartos, J.S. et al., “Interleukin-17 production in central nervous system infiltrating T cells and glial cells is associated with active disease in multiple sclerosis”, Am. J. Pathol., 172:146-155 (2008)). The mRNA levels of p19, p40, and p35 in active SLE patients were also shown to be significantly higher compared with those in inactive SLE patients (Huang, X. et al., “Dysregulated expression of interleukin-23 and interleukin-12 subunits in systemic lupus erythematosus patients”, Mod. Rheumatol., 17:220-223 (2007)), and T cells from lupus patients have a predominant Th1 phenotype (Tucci, M. et al., “Overexpression of interleukin-12 and T helper 1 predominance in lupus nephritis”, Clin. Exp. Immunol., 154:247-254 (2008)).
 Moreover, genome-wide association studies have identified a number of loci associated with chronic inflammatory and autoimmune diseases that encode factors that function in the IL-23 and IL-12 pathways. These genes include IL23A, IL12A, IL12B, IL12RB1, IL12RB2, IL23R, JAK2, TYK2, STAT3, and STAT4 (Lees, C.W. et al., “New IBD genetics: common pathways with other diseases”, Gut, 60:1739-1753 (2011); Tao, J.H. et al., “Meta-analysis of TYK2 gene polymorphisms association with susceptibility to autoimmune and inflammatory diseases”, Mol. Biol. Rep., 38:4663-4672 (2011); Cho, J.H. et al., “Recent insights into the genetics of inflammatory bowel disease”, Gastroenterology, 140:1704-1712 (2011)).
 Indeed, anti-p40 treatment, which inhibits both IL-12 and IL-23, as well as IL-23-specific anti-p19 therapies have been shown to be efficacious in the treatment of autoimmunity in diseases including psoriasis, Crohn’s Disease and psoriatic arthritis (Leonardi, C.L. et al., “PHOENIX 1 study investigators. Efficacy and safety of ustekinumab, a human interleukin-12/23 monoclonal antibody, in patients with psoriasis: 76-week results from a randomized, double-blind, placebo-controlled trial (PHOENIX 1)″, Lancet, 371:1665-1674 (2008); Sandborn, W.J. et al., “Ustekinumab Crohn’s Disease Study Group. A randomized trial of Ustekinumab, a human interleukin-12/23 monoclonal antibody, in patients with moderate-to-severe Crohn’s disease”, Gastroenterology, 135:1130-1141 (2008); Gottlieb, A. et al., “Ustekinumab, a human interleukin 12/23 monoclonal antibody, for psoriatic arthritis: randomized, double-blind, placebo-controlled, crossover trial”, Lancet, 373:633-640 (2009)). Therefore, agents which inhibit the action of IL-12 and IL-23 may be expected to have therapeutic benefit in human autoimmune disorders.
 The Type I group of interferons (IFNs), which include the IFNα members as well as IFNβ, IFNε, IFNκ and IFNω, act through a heterodimer IFNα/β receptor (IFNAR). Type I IFNs have multiple effects in both the innate and adaptive immune systems including activation of both the cellular and humoral immune responses as well as enhancing the expression and release of autoantigens (Hall, J.C. et al., “Type I interferons: crucial participants in disease amplification in autoimmunity”, Nat. Rev. Rheumatol., 6:40-49 (2010)).
 In patients with systemic lupus erythematosus (SLE), a potentially fatal autoimmune disease, increased serum levels of interferon (IFN)α (a type I interferon) or increased expression of type I IFN-regulated genes (a so-called IFNα signature) in peripheral blood mononuclear cells and in affected organs has been demonstrated in a majority of patients (Bennett, L. et al., “Interferon and granulopoiesis signatures in systemic lupus erythematosus blood”, J. Exp. Med., 197:711-723 (2003); Peterson, K.S. et al., “Characterization of heterogeneity in the molecular pathogenesis of lupus nephritis from transcriptional profiles of laser-captured glomeruli”, J. Clin. Invest., 113:1722-1733 (2004)), and several studies have shown that serum IFNα levels correlate with both disease activity and severity (Bengtsson, A.A. et al., “Activation of type I interferon system in systemic lupus erythematosus correlates with disease activity but not with antiretroviral antibodies”, Lupus, 9:664-671 (2000)). A direct role for IFNα in the pathobiology of lupus is evidenced by the observation that the administration of IFNα to patients with malignant or viral diseases can induce a lupus-like syndrome. Moreover, the deletion of the IFNAR in lupus-prone mice provides high protection from autoimmunity, disease severity and mortality (Santiago-Raber, M.L. et al., “Type-I interferon receptor deficiency reduces lupus-like disease in NZB mice”, J. Exp. Med., 197:777-788 (2003)), and genome-wide association studies have identified loci associated with lupus that encode factors that function in the type I interferon pathway, including IRF5, IKBKE, TYK2, and STAT4 (Deng, Y. et al., “Genetic susceptibility to systemic lupus erythematosus in the genomic era”, Nat. Rev. Rheumatol., 6:683-692 (2010); Sandling, J.K. et al., “A candidate gene study of the type I interferon pathway implicates IKBKE and IL8 as risk loci for SLE”, Eur. J. Hum. Genet., 19:479-484 (2011)). In addition to lupus, there is evidence that aberrant activation of type I interferon-mediated pathways are important in the pathobiology of other autoimmune diseases such as Sjogren’s syndrome and scleroderma (Båve, U. et al., “Activation of the type I interferon system in primary Sjogren’s syndrome: a possible etiopathogenic mechanism”, Arthritis Rheum., 52:1185-1195 (2005); Kim, D. et al., “Induction of interferon-alpha by scleroderma sera containing autoantibodies to topoisomerase I: association of higher interferon-alpha activity with lung fibrosis”, Arthritis Rheum., 58:2163-2173 (2008)). Therefore, agents which inhibit the action of type I interferon responses may be expected to have therapeutic benefit in human autoimmune disorders.
 Tyrosine kinase 2 (Tyk2) is a member of the Janus kinase (JAK) family of nonreceptor tyrosine kinases and has been shown to be critical in regulating the signal transduction cascade downstream of receptors for IL-12, IL-23 and type I interferons in both mice (Ishizaki, M. et al., “Involvement of Tyrosine Kinase-2 in Both the IL-12/Th1 and IL-23/Th17 Axes In vivo”, J. Immunol., 187:181-189 (2011); Prchal-Murphy, M. et al., “TYK2 kinase activity is required for functional type I interferon responses in vivo”, PLoS One, 7:e39141 (2012)) and humans (Minegishi, Y. et al., “Human tyrosine kinase 2 deficiency reveals its requisite roles in multiple cytokine signals involved in innate and acquired immunity”, Immunity, 25:745-755 (2006)). Tyk2 mediates the receptor-induced phosphorylation of members of the STAT family of transcription factors, an essential signal that leads to the dimerization of STAT proteins and the transcription of STAT-dependent pro-inflammatory genes. Tyk2-deficient mice are resistant to experimental models of colitis, psoriasis and multiple sclerosis, demonstrating the importance of Tyk2-mediated signaling in autoimmunity and related disorders (Ishizaki, M. et al., “Involvement of Tyrosine Kinase-2 in Both the IL-12/Th1 and IL-23/Th17 Axes In vivo”, J. Immunol., 187:181-189 (2011); Oyamada, A. et al., “Tyrosine kinase 2 plays critical roles in the pathogenic CD4 T cell responses for the development of experimental autoimmune encephalomyelitis”, J. Immunol., 183:7539-7546 (2009)).
 In humans, individuals expressing an inactive variant of Tyk2 are protected from multiple sclerosis and possibly other autoimmune disorders (Couturier, N. et al., “Tyrosine kinase 2 variant influences T lymphocyte polarization and multiple sclerosis susceptibility”, Brain, 134:693-703 (2011)). Genome-wide association studies have shown other variants of Tyk2 to be associated with autoimmune disorders such as Crohn’s Disease, psoriasis, systemic lupus erythematosus, and rheumatoid arthritis, further demonstrating the importance of Tyk2 in autoimmunity (Ellinghaus, D. et al., “Combined Analysis of Genome-wide Association Studies for Crohn Disease and Psoriasis Identifies Seven Shared Susceptibility Loci”, Am. J. Hum. Genet., 90:636-647 (2012); Graham, D. et al., “Association of polymorphisms across the tyrosine kinase gene, TYK2 in UK SLE families”, Rheumatology (Oxford), 46:927-930 (2007); Eyre, S. et al., “High-density genetic mapping identifies new susceptibility loci for rheumatoid arthritis”, Nat. Genet., 44:1336-1340 (2012)).
 In view of the conditions that may benefit by treatment involving the modulation of cytokines and/or interferons, new compounds capable of modulating cytokines and/or interferons, such as IL-12, IL-23 and/or IFNα, and methods of using these compounds may provide substantial therapeutic benefits to a wide variety of patients in need thereof.
 U.S. Pat. No. 9,663,467 discloses compounds useful for treating diseases and conditions related to the modulation of IL-12, IL-23 and/or IFNα such as autoimmune and inflammatory diseases. This reference is assigned to the present assignee and is incorporated herein by reference in its entirety.
 The compound shown below which is disclosed therein
has shown activity against the targets.
 U.S. Pat. No. 9,505,748 also discloses compounds useful for treating diseases and conditions related to the modulation of IL-12, IL-23 and/or IFNα such as autoimmune and inflammatory diseases. This reference is assigned to the present assignee and is incorporated herein by reference in its entirety.
 The compound shown below which is disclosed therein
has shown activity against the targets.
 As may be appreciated, there remains a need for improved delivery of the above compounds to the patient.
 Applicants have found prodrugs of Compounds (I) and (II) useful for the administration of Compounds (I) and (II), respectively. The prodrugs have better solubility at physiological important pH values than Compounds (I) and (II), and surprisingly allow the administration of Compounds (I) and (II) with a wider dosage range and/or a broader range of pharmaceutical formulations. These prodrug compounds are provided to be useful as pharmaceuticals with desirable stability, bioavailability, therapeutic index, and toxicity values that are important to their drugability.
SUMMARY OF THE INVENTION
 The invention is directed to prodrugs of Compounds (I) and (II) which are useful as modulators of IL-12, IL-23 and/or IFNα by inhibiting Tyk2-mediated signal transduction.
 The present invention also provides processes and intermediates for making the compounds of the present invention.
 The present invention also provides pharmaceutical compositions comprising a pharmaceutically acceptable carrier and at least one of the compounds of the present invention.
 The present invention also provides a method for the modulation of IL-12, IL-23 and/or IFNα by inhibiting Tyk-2-mediated signal transduction comprising administering to a host in need of such treatment a therapeutically effective amount of at least one of the compounds of the present invention.
 The present invention also provides a method for treating proliferative, metabolic, allergic, autoimmune and inflammatory diseases, comprising administering to a host in need of such treatment a therapeutically effective amount of at least one of the compounds of the present invention.
 A preferred embodiment is a method for treating inflammatory and autoimmune diseases or diseases. For the purposes of this invention, an inflammatory and autoimmune disease or disorder includes any disease having an inflammatory or autoimmune component.
 An alternate preferred embodiment is a method for treating metabolic diseases, including type 2 diabetes and atherosclerosis.
 The present invention also provides the use of the compounds of the present invention for the manufacture of a medicament for the treatment of cancers.
 The present invention also provides the compounds of the present invention for use in therapy.
 These and other features of the invention will be set forth in the expanded form as the disclosure continues.