Intent to Treat

In 1999 Lynn Sakia and colleagues suggested the fibrillin 1 protein was related to latent Transforming Growth Factor beta (TGFb) binding protein (LTBP) based on sequence similarity. This association suggested to the investigators that fibrillin 1 may play a role in regulating levels of TGFb potentially implicating the hormone in the pathophysiology of Marfan Syndrome (MFS). Their paper lead to a series of studies concluding TGFb played a major role in the pathogenesis of MFS.  Included among the follow-up studies was the observation that transgenic mice carrying one mutant copy of the fibrillin 1 gene and a copy of the wild-type mouse fibrillin 1 gene had elevated TGFb levels and an increase in phosphoSMAD2/3 (pSMAD2/3), the activated form of one of the substrates of the TGFb receptor (TGFbR).  More compelling was the mitigation of the vascular pathology characteristic of the mouse disease model when mutant mice were given an anti-TGFb antibody; treated animals did not show progression of aortic dilation that developed in untreated “Marfan mice”.  The association of aortic disease with mutations in the TGFb receptors I and II further strengthened the thesis that a constitutively activated TGFb pathway causes this type of disease and that targeting the TGFb pathway could help patients with MFS and related diseases. 

Predating the TGFb work by a decade were the clinical observations that pathologically elevated angiotensin II levels were partially responsible for cardiac, renal and vascular damage secondary to ischemia and hypertension, specifically intimal hyperplasia, fibrosis and hypertrophy.  Angiotensin II (ATII) is a vasoactive substance in the renin-angiotensin-aldosterone system with a variety of physiological activities including the induction of vasoconstriction, cell proliferation (e.g. vascular smooth muscle cells and fibroblasts), production of extracellular matrix (ECM) and aldosterone release.  The pro-inflammatory actions of ATII have been shown to play a central role in pathogenesis of atherosclerotic vascular disease.  Most of these actions are mediated through the angiotensin II Type 1 (AT1) receptor, a G-protein coupled receptor expressed in a variety of cell types including endothelial, central nervous system cells and vascular smooth muscle cells.

Angiotensin II is a peptide produced through the cleavage of a larger protein by the enzyme angiotensin converting enzyme (ACE).  Inhibitors of ACE have been conclusively shown to modify pathologic cardiac remodeling after ischemic myocardial damage and to protect kidney from hypertensive damage.  Laboratory evidence that the ATII Type IA and B receptors may be the preferred clinical target and the clinical benefit from the use of ACE inhibitors lead to the development of angiotensin II Type I receptor antagonists (ARA). 

The angiotensin II and TGFb pathways interact at several levels.  Angiotensin II both induces the expression of the TGFb gene and induces the release of TGFb from its latent form in a process thought to be mediated in part by the induction of proteases MMP2 and MMP9 and thrombospondin 1 (TSP1), a multifunctional extracellular matrix protein.  By this mechanism angiotensin II can affect TGFb-regulated genes directly through TGFb but angiotensin II also activates SMAD2/3 through a mechanism independent of TGFb: angiotensin II induces SMAD2/3 phosphorylation through one or more pathways.  Those pathways could potentially include p38-MAPK, ERK1/2, JAK/STAT, nuclear factor-kB (NF-kB), JNK, Ras and Rap1 pathways or beta-arrestin all of which in various cell type mediate the effects of ATII receptor activation.  Crosstalk between ATII receptor signaling and the TGFb signaling could provide an additional mechanism of action for some of the beneficial effects on cardiac architecture post-injury and the reno-protectiveness of the ARAs.  Specifically, blockade of this receptor, ATII Type1, has anti-mitotic and anti-inflammatory effects. The simplest hypothesis regarding the beneficial action of ARAs may be their anti-inflammatory activities given that AII initiates inflammatory cascades in endothelial and vascular smooth muscle cells.  Perhaps not surprisingly, the vascular histopathology associated with elevated angiotensin II shares some features with that found for the Marfan Syndrome.  This raises an obvious question: does TGFb pathway activation activate some of the ATIIRI intracellular mediators? Asked differently, are some of the pathologic changes typical of MFS and Loeys Dietz Syndrome owing to ATII-regulated processes via the TGFb pathway?   The enormous complexities of ATIR and TGFbR signaling underscore the many unknowns regarding the molecular details of TGFb-mediated pathology and its treatment.

Beyond the details of how angiotensin receptor blockade effects its beneficial actions in models of MFS, there is the practical question of which of the ARAs are best for the treatment of the TGFb-opathies and for each ARA, what is the correct dose?  There are 7 members of this class of drugs on the market, losartan being the oldest and least ATIIR Type I-specific.  Losartan has the shortest half-life at about 6-9 hours; telmisartan has a half-life of 24 hours but shows 3000-fold greater specificity for AT1 over AT2 receptors compared to losartan's 1000-fold specificity.  One could make the case that QD dosing of losartan has proved itself clinically in reducing what could be imputed to be SMAD2/3-mediated pathologic processes. Clinically, the “biomarker” is blood pressure with dose being titrated to a tolerable lowering of diastolic pressure.   Is blood pressure the right marker for TGFb pathway down-regulation given that mechanistically blood pressure reduction is not known to be mediated through the TGFb pathway?  Or is the question moot because orthostasis is always dose-limiting?  The later seems less likely because at maximum recommended doses of losartan, the blood pressure of a normotensive subject is often not significantly changed.  

Concomitant with clinical evidence that ARAs are disease-modifying in the TGFb-opathies, the field needs a reliable marker for measuring therapeutic benefit.  The ideal biomarker would be an analyte that is easy to access, for example, from blood, an analyte that is a participant in the pathological process, an analyte that while circulating reflects the pathological processes in the relevant diseased tissues.  There are a variety of conceivable biomarkers including phospho-SMAD2/3, other proteins in the intracellular pathways, and the proteins they ultimately regulate such as thrombospondin 1.  But much of the pathology of these diseases is local – in extracellular matrix, vascular smooth muscle cells, infiltrating lymphocytes; the biomarker must reliably read on local biology, a greater burden of proof than typical for biomarkers.  In the absence of such a marker, under-dosing risks a negative result, i.e. no benefit.  Likewise, long-term over-dosing is always a concern -- what, for example, is the effect of TGFb modulation on Th1 or TH17 differentiation – though there is an abundance of data supporting long-term safety of ARAs.   

Would ARAs enhance skeletal muscle growth?  There are reasons to believe they might.  First, blockade of myostatin or ablation of the myostatin genes have been shown increase skeletal muscle mass in mice. Second, antibodies to TGFb given to the Marfan mouse with defective muscle regeneration improve muscle strength , mass and satellite cell proliferation.  Third, the myostatin receptors use the same SMAD2/3 signaling pathway demonstrated to be down-regulated by ARAs. Fourth, long-term treatment with losartan of the Marfan mouse (Fbn1 C1039G/+) "fully normalized steady state muscle architecture".  Fifth, there is clinical evidence of increases in skeletal muscle mass in cardiomyopathic patients treated with ARAs.  The molecular, cellular and clinical data all support the expectation that ARAs can enhance skeletal muscle mass.

Recommendations in medicine are rarely dictated by the proof even good evidence affords.  Indeed, most of the common medical practices have not been subject to rigorous clinical trial.  And rarely is all the information at hand – there are always more tests that could be done, more time to pass before certainty sets in.  Patients and physicians do not have the luxury of completeness.  The decision to treat is often about the calculus of risk and reward.  Patients with their physicians collaborate; they teach each other; they calibrate one another to understand the meaning and likelihood of true benefit and real danger.  There is cost, convenience, comfort, and a hundred other considerations.  By necessity, each decision is unique and crafted for the circumstances.   

My greatest concern regarding my daughter’s health is the possibility of the vascular disease associated with Marfan Syndrome and Loeys Dietz Syndrome.  Without a diagnosis, it is still a very real possibility.  The vascular pathology of this family of conditions is, for now, irreversible.  The ill-formed and injured great vessels do not heal themselves.   Therefore, the decision not to treat potentially allows the damage to proceed ineluctably.  On the other hand, the decision to stop treatment can be made at any time.  There is the added but clinically unproven prospect ARAs enhance muscle growth.  The adverse events caused by this class of drugs are largely benign and reversible; it does not feel like much of a choice.  

Hypotheses that need testing: 

1) ARAs will have a beneficial effect on the evolution of typical vascular disease in MFS and LDS at a dose that does not lower blood pressure excessively.

        This has recently been published in the New England Journal of Medicine in the June 26 2008 issue (NEJM 358, 2787, 2008).   In a small study of patients who progressed on one of the standard regimens -- beta blockers, ACE inhibitors and Ca channel blockers -- in these patients the drug stabilized the growth of their aortic root and in some cases reduced (relatively) the root as they grew.  This is extremely good news for patients with TGFb-opathies such as Marfan and Loyes Dietz Syndromes.  The drugs will likely have an effect on other aspects of the biology of Marfan Syndrome affected by TGF beta signaling including their muscle pathology.

2) ARAs will have a beneficial effect on muscle physiology increasing muscle mass and promoting normal muscle physiology including healing.

        I have heard through the proverbial medical grapevine that patients treated with ARA's have shown increased muscle mass and strength.  Nothing formally is published.   

3) ARAs will have less of an effect on skeletal development such as long bone growth and kyphoscoliosis.

4) The local effect ARAs have on cell types that are the primary effectors of vascular disease secondary to elevated TGFb signaling primary can be measured in the circulation. 

        TGF beta levels have been measured in patients receiving renal transplants and serum TGF beta levels drop 50% in two on losartan.  It may be possible to titrate doses to lower TGF beta levels.