Abstract Our long-term objective is to develop an effective treatment for scleroderma (systemic sclerosis, SSc) skin fibrosis. Caveolin-1 is a promising therapeutic target in fibrotic diseases. The profibrotic effects of caveolin-1 deficiency in cells and in mouse models is suppressed by a peptide equivalent to its active site (caveolin-1 scaffolding domain, CSD). However, CSD lacks suitable pharmacologic properties for drug development. To overcome this problem, we developed novel, modified versions of CSD. We first divided CSD into three subregions and found that all three suppressed bleomycin-induced skin fibrosis. To improve, the pharmacological properties, we then modified CSD and each subregion to be water soluble and protected from proteolysis. This modification greatly enhanced the uptake by cells of all four modified peptides and also greatly increased their ability to inhibit several purified kinases in vitro. We have so far tested only one of the four modified peptides in vivo and it was outstandingly active in inhibiting bleomycin-induced dermal fibrosis as well as the associated loss of the intradermal adipose layer (lipoatrophy). These initial studies justify and outstandingly support our proposal to identify a Lead Compound from among the four candidates, then evaluate its Therapeutic Index (ratio between toxic and beneficial doses). Our studies and the literature also suggest that our peptides will be more effective and have fewer side effects than the blockbuster FDA- approved drug pirfenidone (brand name Esbriet). In summary, to proceed with drug development we must identify a Lead Compound. Due to their distinct pharmacological and functional differences, we must do a side- by-side comparison of our four modified peptides. 1) Select a Lead Compound using two model systems: Systemic Bleomycin Treatment and Subcutaneous TGFβ Injection. We will identify a Lead Compound, then demonstrate its specificity and activity by comparing it to a control peptide (scrambled Lead) and to nintedanib. Peptides will be delivered s.c. in a Therapeutic Protocol, beginning one week after fibrosis is induced. Primary Readouts will be dermal fibrosis and lipoatrophy. Secondary Readouts will be the levels of markers for myofibroblasts, adipocytes, and inflammatory cells. Success will be defined as >50% reversal of the deleterious effects of bleomycin and TGFβ on the Primary and Secondary Readouts. 2) Determine the Therapeutic Index of the Lead Compound. The dose-dependence of the beneficial effects of the Lead Compound will be determined using doses above and below our current standard dose. The toxicity of the Lead Compound will be evaluated in a Single-Treatment Maximum Tolerated Dose (MTD) Experiment using 1X, 5X, 25X, and 125X our current standard dose. We will consider these studies to be a success if the Therapeutic Index is >50. In summary, these studies will provide a novel Lead Compound that meets our Criteria for Success, both in terms of sup...