The phenomenon of nephrolithiasis afflicts approximately 10% of the population globally, and its prevalence is rising continuously [1, 2]. Of the main types of kidney stones—calcium oxalate (CaOx), uric acid, struvite, hydroxyapatite, brushite and cystine—calcium-containing stones are by far the most prevalent, comprising as much as 80% of all stones.
Kidney stones have been the target for chemolysis for many years though with only limited success. Of the various kidney stones, CaOx stones in particular are considered to be the most resistant to chemolysis. Studies of oral intake treatments find little evidence of successful dissolution of existing CaOx stones (or, even of inhibiting stone formation [3,4,5]). In vivo chemolysis of kidney and bladder stones began seriously with the use of Renacidin, which appeared effective as a powerful dissolution agent for calcium phosphate, calcium carbonate, and magnesium ammonium phosphate (struvite) stones, but not for CaOx stones ; see also, e.g.,  for calcium phosphate stones. If appropriate agents could be identified, the principal of in vivo chemolysis of CaOx kidney stones remains attractive [8, 9].
In efforts to investigate chemolytic behavior of CaOx stones, several in vitro studies have considered various potential agents. These agents have generally demonstrated poor efficacy in terms of percentage dissolved and relatively long times required for disintegration, for both real and synthetic stone types. Studies using artificial, synthesized “kidney stones”, consisting of relatively pure calcium oxalate aggregates and calcium phosphate crystals [9,10,11], yielded disappointing results of up to 10% stone weight loss over several hours, and 13–47% stone weight loss over days using enzymatic disintegration (oxalate decarboxylase and oxalate oxidase) to attack oxalate . Moreover, in vitro studies that test chemolysis of real CaOx kidney stones harvested from human patients are similarly limited, particularly in terms of (1) the actual number of studies, and (2) the small number and type of chemolysis agents and stones tested in each study. For example, one report focused mostly on snake venom thrombin-like enzyme with the addition of antibiotics , while another  investigated natural and synthetic chelating reagents (citrate and EDTA) together with an antibiotic. These studies on real stones, too, demonstrated overall poor efficacy in terms of percentage disintegrated and/or time required for disintegration, e.g., ~ 10–50% disintegration by weight after 5 days , and up to ~ 10% over several (2–10) hours .
Significantly, though, with the partial exception of one investigation of an enzyme to specifically target proteins in the stone matrix , essentially all of these studies investigate agents that are expected to attack the CaOx (inorganic) components of a stone. And yet, a range of studies have identified significant organic matter, even as a matrix structure, within these stones, which is presumed to bind CaOx particles and aggregates [13,14,15,16,17,18,19,20,21,22,23,24,25]. Indeed, scanning electron microscope images (from these and other studies) show that CaOx stones are highly heterogeneous, containing layered, amorphous, and crystalline material. In this context, too, over 1000 proteins have been identified in kidney stones , which may form an organic protein matrix [13, 14]. However, while the literature mentions the presence of organic matter, it does not generally give quantitative measures of the relative amount of organic matter [13,14,15,16,17,18,19,20,21]; one series of studies yielded estimates of ~ 2.5% organic matrix by weight [22, 23]. Identification and quantification of organic constituents, whether from surface analysis or total sample analysis, remains difficult. Moreover, the precise nature and influence of organic matter remains uncertain, and the amount and type of organic matter varies among different stones, patient pathologies, and other factors. Given the assumption that the organic “matrix” in stones may act to bind inorganic components (such as CaOx particles), the overall stone structure may be that of a “brick and mortar” configuration, wherein the organic matter is the “mortar” and the inorganic constituents being the “bricks”; if so, then regardless of the specific stone configuration (e.g., containing layered, amorphous, and/or crystalline regions), it might be possible to achieve fast stone disintegration by attacking organic components.
In light of the highly limited information available on disintegration properties of real CaOx kidney stones, and to evaluate potential new routes for, ultimately, in vivo chemolytic treatments, the objectives of this study were to systematically analyze potential chemolysis agents on real kidney stones, effective within hours, and without regard to immediate clinical application. For the first time, we focus specifically on the question of whether CaOx stones can be treated as “brick and mortar” aggregates, which would enable chemolysis targeted to attack the organic material within CaOx stones [13,14,15,16,17,18,19,20,21,22,23,24,25]. We note that in all such studies, it is important to differentiate between “dissolution” and “disintegration”—while pure synthetic CaOx particles dissolve to various extents, CaOx kidney stones (which are aggregates of inorganic and organic materials) may also disintegrate into fragments when binding material is affected—so that the term “chemolysis” encompasses all processes of stone decomposition.