BMP-7 (bone morphogenetic protein 7), also known as OP-1 (osteogenic protein-1), is a secreted growth factor of the TGF-β superfamily. The distinction matters: BMP-7 is not a short synthetic peptide. The biologically active species is a disulfide-linked, glycosylated homodimer, and it is the dimer, not the individual chain, that carries activity. Its size, three-dimensional structure, manufacture, stability and analytical evaluation require a different approach from a peptide [14].

The only molecule in this group that reached the operating room

Most proteins discussed in an anti-fibrotic context never left the laboratory. BMP-7 is the exception, and an unusual one: for more than a decade it was a product a surgeon could order by catalogue number, but not as an anti-fibrotic. Under the name OP-1, recombinant human BMP-7 on a collagen carrier was authorised by the FDA (US Food and Drug Administration) under a humanitarian device exemption (HDE): in 2001 for long-bone nonunions, later for revision posterolateral spinal fusion. The registration study in tibial nonunions compared OP-1 against autograft [6][10].

In other words, BMP-7's clinical history happened in orthopaedics while its scientific reputation was built in nephrology, and the two lines barely intersected. Understanding that gap matters more than any single rodent number.

Two names for one molecule

BMP-7 and OP-1 are the same BMP7 gene product. The naming split dates to two parallel cloning efforts in the late 1980s: the Wozney group characterised the BMP family from bone-inductive extract [1][2], while the Ozkaynak group independently cloned the OP-1 cDNA and placed it in the TGF-β superfamily [3].

The practical consequence: orthopaedic and device literature says OP-1; nephrology and developmental biology say BMP-7. Searching under only one name systematically loses half the sources, and reagent catalogues may use either.

A brake on TGF-β1, not a repair signal

This is the substantive difference between BMP-7 and molecules conventionally described as "regeneration signals".

TGF-β1 signals through TGF-β receptors to Smad2/3, the profibrotic arm, central to fibrosis in essentially any organ [15]. BMP-7 binds BMP receptor complexes (primarily ALK3/BMPR-IA) and activates Smad1/5/8; phosphorylated Smad1/5/8 forms a complex with Smad4 and affects transcription [14]. The two arms compete for shared Smad4 and mutually repress one another. BMP-7 is therefore more accurately described not as a stimulus to repair but as an endogenous brake that continuously holds the profibrotic arm in check.

A non-trivial consequence follows. In the healthy kidney BMP-7 is constitutively expressed. After ischaemic, toxic or metabolic injury its expression falls, and fibrosis proceeds not because a new stimulus appeared but because restraint was removed. "Giving BMP-7" is therefore an attempt to restore a brake, not to add a signal. Whether a restored brake can resolve established scar is a fundamentally different question from whether it can prevent new scar, and the answers do not coincide.

Kidney models: from ischaemia to reversal of EMT

The preclinical base here is genuinely large and reproducible. In rats, OP-1 given in ischaemic acute renal failure reduced injury severity [4]. In unilateral ureteral obstruction, OP-1 prevented fibrogenesis [5]. The most-cited work is Zeisberg and colleagues in Nature Medicine: BMP-7 counteracted TGF-β1-induced epithelial-to-mesenchymal transition (EMT) in renal tubular epithelial cells, re-induced E-cadherin, and in mouse models was reported to reverse established chronic renal injury [8]. Later work continued to look for ways to deliver that effect, for example through micellised transduction-domain constructs [17].

What has to be said honestly. First, the quantitative contribution of EMT to adult kidney fibrosis became the subject of a long separate debate after 2003 and is assessed more conservatively today than at the time of publication. Second, these are rodents, and the doses used were supraphysiological. Third, none of these models is human chronic kidney disease. Results from cell, mouse or pig models are not clinical confirmation in people.

Why "add BMP-7" is not the same as "release the brake"

Ligand abundance is only part of what sets BMP signalling. A layer of endogenous antagonists that bind BMPs extracellularly: USAG-1 (Sostdc1), gremlin, noggin. USAG-1 is expressed in distal tubules and negatively regulates specifically the renoprotective action of BMP-7; USAG-1-null mice are protected from tubular injury [9].

That means the pathway's set-point is governed by the ligand/antagonist ratio, not by absolute ligand level. In injured tissue antagonists are frequently upregulated, so delivering exogenous BMP-7 and removing an antagonist are not equivalent interventions. This substantially explains why preclinical doses had to be kept high: part of the delivered protein is neutralised before it ever reaches a receptor.

The carrier decides everything: implant versus injection

Clinically, BMP-7 never worked as a systemic injection. Its clinical form was a solid implant: lyophilised rhBMP-7 loaded onto a type-I collagen matrix and placed directly into the defect. The reason lies in the molecule's own pharmacology: BMPs bind extracellular matrix avidly, are poorly retained at the application site, and have an unfavourable systemic exposure profile; retention at the application site was analysed as a problem in its own right as early as 2001 [7].

Compounding this, BMP receptors are near-ubiquitously expressed. A dose sufficient to reach renal receptors systemically simultaneously acts on bone and other tissues, hence concern about heterotopic ossification. It is no accident that subsequent development effort went not toward new doses but toward carriers: Vukicevic and colleagues return explicitly to carrier design as the limiting factor in clinical BMP use [13].

The practical conclusion: the orthopaedic product could not simply be redirected at the kidney. These were different delivery problems. It also means that not every product labelled BMP-7 has the same properties, construct, expression source, carrier and material form determine the outcome.

How an authorised product left the market

Precision is needed here, because the story is easy to misstate in either direction.

OP-1 never held a full PMA approval. It existed under an HDE, a pathway for devices intended for small populations, with lower efficacy-evidence requirements than a PMA. In 2009 an FDA advisory committee voted against expanding OP-1 Putty to uninstrumented posterolateral lumbar fusion. In 2010 Stryker Biotech sold the OP-1 assets to Olympus (Olympus Biotech); in 2014 Olympus halted the effort and closed its US operations, after which rhBMP-7 was no longer marketed in the United States. The assets subsequently changed hands again.

What should not be inferred: that BMP-7 was found unsafe. The market exit was a commercial and regulatory-pathway outcome, a narrow indication, a restrictive authorisation route, a refused expansion, a change of owner, not a finding of demonstrated harm. Nor should it be minimised: the product is genuinely no longer available.

Two stories that are frequently conflated should also be separated. The prominent safety controversy over BMPs in spinal surgery concerned primarily rhBMP-2, a different protein, a different manufacturer, a different product [11]. Transferring those conclusions to BMP-7 is not correct.

ALK3 mimetics, and the phase 2 that did not work

Since the protein itself could not be delivered systemically, the field tried to bypass it. Sugimoto and colleagues showed that the ALK3 receptor is critical for kidney regeneration and that a small peptide ALK3 agonist (THR-123), suitable for systemic administration, reversed established fibrosis in mice [12]. On that basis the clinical candidate THR-184 was developed; the design methodology for such mimetics is described separately [18].

The outcome is known and there is no point softening it. In a randomised, placebo-controlled phase 2 trial in patients at high risk of acute kidney injury after cardiac surgery, THR-184 across a range of dose exposures did not reduce the incidence, severity or duration of AKI compared with placebo; AKI occurred in roughly 74–79% of patients across all groups, placebo included. Safety outcomes were comparable between groups [16].

That is where matters stand: reproducible anti-fibrotic rodent data, a clear and elegant mechanism, and one properly controlled human trial of the approach, negative. No approved BMP-7-based anti-fibrotic product exists.

What to check in an analytical package

For a short peptide, HPLC (high-performance liquid chromatography) and mass spectrometry are often enough as a core pair. A recombinant protein needs a broader set of checks, and several points here are specific to BMP-7:

  • Dimer, not monomer. Activity resides in the disulfide-linked homodimer; the reduced monomer is inactive. Disulfide-bond state is not a formality, it is the most common cause of a "silent" reagent.
  • Expression system. Mammalian-cell (CHO) material is glycosylated; E. coli material is not and requires refolding. They are not interchangeable in bioassays by default.
  • Mature protein versus proprotein. BMP-7 is synthesised as a precursor; the specification should state the mature domain unambiguously.
  • Aggregate profile and purity. For a protein of this class, aggregation affects both assay result and reproducibility.
  • Activity in a relevant assay. Identity by mass does not substitute for functional confirmation.
  • Adsorption. The molecule binds plastic and matrix avidly, at low concentrations this measurably affects the actually available concentration.
  • Two names. Literature and documentation searches should cover both BMP-7 and OP-1.

The exact package depends on the construct and material form; evaluating a specific lot requires its specification and COA (certificate of analysis).

Material status

BMP-7 is supplied as a research-use-only (RUO) reagent. It is not a medicine, not a supplement, and not a product for human consumption. No approved anti-fibrotic indication for BMP-7 exists in any jurisdiction; the previously marketed orthopaedic product OP-1 has been withdrawn from the US market. The material is intended for in vitro laboratory research and preclinical models. This monograph describes the biological object and published research directions, it is not a use protocol.

The BMP-7 catalogue item is currently coming soon. Related material: HGF (the c-Met ligand, unlike BMP-7, a reparative signal rather than a brake), Ac-SDKP (a peptide inhibitor of the Smad2 arm), and the tissue repair catalogue section.