Demonstrating biosimilarity entails comprehensive analytical evaluations, clinical pharmacolo-gy profiling, and efficacy testing for at least one medical indication in patients. These require-ments are stipulated by the U.S. Biologics Price Competition and Innovation Act (BPCIA). The costliest element—efficacy testing—can be waived if other compliance benchmarks are satisfied, including comparing functional pharmacodynamic (PD) biomarkers, even when they do not di-rectly correlate with clinical outcomes. Most biological drugs, such as monoclonal antibodies (mAbs), lack identifiable PD biomarkers. The FDA has employed various 'omics' technologies to identify potential PD biomarkers, including proteomics, glycomics, transcriptomics, genomics, epigenomics, and metabolomics. Although these efforts provide a robust scientific basis for estab-lishing biosimilarity, they are neither practical nor necessarily superior to existing functional biomarkers, such as receptor binding and mode-of-action outcomes. As we report for the first time, these functional biomarkers can effectively serve as PD indicators for all FDA-licensed bio-logical drugs. We recommend that the FDA consider officially listing these functional biomarkers to expedite and reduce the cost of biosimilar development, thereby increasing the accessibility of biological drugs. PD surrogates, like the receptor binding and pharmacokinetic profiles, are more robust and offer a rational solution to finding PD markers to compare for establishing biosimi-larity.

Objectives: To investigate the comparative antiseizure activity of the l-enantiomers of d,l-fenfluramine and d,l-norfenfluramine and to evaluate the relationship between their concentration in plasma and brain and anticonvulsant activity. Methods: d,l-Fenfluramine, d,l-norfenfluramine and their individual enantiomers were evaluated in the mouse maximal electroshock seizure (MES) test. d,l-Fenfluramine, d,l-norfenfluramine and their individual l-enantiomers were also assessed in the DBA/2 mouse audiogenic seizure model. All compounds were administered intraperitoneally. Brain and plasma concentrations of the test compounds in DBA/2 mice were quantified and correlated with anticonvulsant activity. Results: In the MES test, fenfluramine, norfenfluramine and their enantiomers showed comparable anticonvulsant activity, with ED50 values between 5.1 and 14.8 mg/kg. In the audiogenic seizure model, l-norfenfluramine was 9 times more potent than d,l-fenfluramine and 15 times more potent than l-fenfluramine based on ED50 (1.2 vs 10.2 and 17.7 mg/kg, respectively). Brain concentrations of all compounds were about 20-fold higher than in plasma. Based on brain EC50 values, l-norfenfluramine was 7 times more potent than d,l-fenfluramine and 13 times more potent than l-fenfluramine (1940 vs 13,200 and 25,400 ng/g, respectively). EC50 values for metabolically formed d,l-norfenfluramine and l-norfenfluramine were similar to brain EC50 values of the same compounds administered as such, suggesting that, in the audiogenic seizure model, the metabolites were responsible for the antiseizure activity of the parent compounds. Significance: Because of the evidence linking d-norfenfluramine to d,l-fenfluramine to cardiovascular and metabolic adverse effects, their l-enantiomers could potentially be safer follow-up compounds to d,l-fenfluramine. We found that in the models tested, l-fenfluramine and l-norfenfluramine were as active as the corresponding racemates. Based on the results in DBA/2 mice and other considerations, l-norfenfluramine appears to be a particularly attractive candidate for further evaluation as a novel, enantiomerically pure antiseizure medication.

Objective: We report a case of acute pancytopenia and liver injury after concomitant administration of low-dose methotrexate (MTX), and high-dose esomeprazole and metamizole. Case summary: A 71-year-old patient with chronic systemic idiopathic erosive arthritis was admitted after a pelvic ring fracture. After hospital admission, she received MTX in addition to esomeprazole and metamizole. Subsequent laboratory tests revealed pancytopenia and elevated liver enzymes. The follow-up clinical examinations were unremarkable, with the exception of sub-febrile temperatures. Further investigations did not detect a definitive etiology. Due to the suspicion of methotrexate-induced hematotoxicity and hepatotoxicity, antagonizing therapy with calcium folinate was administered, after which the blood counts and liver parameters normalized. Discussion: Due to the administration of high-dose metamizole immediately before the administration of low-dose MTX, the pre-existing hematotoxic pharmacodynamic effect of MTX was acutely enhanced by that of metamizole, although folic acid was administered preemptively. In addition, the concomitant administration of high-dose esomeprazole and normal dose torasemide resulted in a pharmacokinetic interaction with MTX by decreasing its renal secretion and elimination, further enhancing the concentration-dependent hematotoxic and hepatotoxic side effects of MTX. Conclusions: The relatively high demand for analgesics in patients with chronic rheumatic diseases already being treated with MTX and proton pump inhibitors necessitates that clinicians consider drug-drug interactions as potential causes of adverse drug reactions after the administration of metamizole or nonsteroidal anti-inflammatory drugs. In this category of patients, it is strongly recommended to switch to analgesics of other classes.

The FDA has concluded that a biosimilar candidate capable of demonstrating pharmacodynamic biomarkers in a healthy subject need not be tested for clinical efficacy in patients, regardless of if the biomarker correlates with clinical response. Since monoclonal antibodies (mAbs) do not trig-ger pharmacodynamic response, they can be substituted with robust functional disqualifying them for this waiver that can be overcome by allowing comparison of functional properties that eventually result in clinical response. This suggestion is based on the FDA's preference for more sensitive testing methods. However, clinical efficacy testing in patients is the least sensitive method, as confirmed by statistical modeling, a fact that regulatory agencies need to admit. In this paper, I present a logical and rational argument to establish the biosimilarity of products that do not have pharmacodynamic biomarkers based on their orthogonally proven functional bio-similarity. This understanding will significantly lower the development cost of biosimilars, a goal that the FDA outlined in all its guidance. Keywords: biosimilarity 1; biosimilars 2; pharmacodynamic biomarkers 3; monoclonal antibodies 4; FDA 5; clinical efficacy testing in healthy subjects 6; clinical efficacy testing in patients 7; functional assays 8; receptor binding 9.