This guidance represents the Food and Drug Administration's (FDA's) current thinking on this topic. It does not create or confer any rights for or on any person and does not operate to bind FDA or the public. You can use an alternative approach if the approach satisfies the requirements of the applicable statutes and regulations. If you want to discuss an alternative approach, contact the FDA staff responsible for implementing this guidance. If you cannot identify the appropriate FDA staff, call the appropriate number listed on the title page of this guidance.I.

OBJECTIVES:Linaclotide is a minimally absorbed guanylate cyclase-C agonist. The objective of this trial was to determine the efficacy and safety of linaclotide in patients with irritable bowel syndrome with constipation (IBS-C).METHODS:This phase 3, double-blind, parallel-group, placebo-controlled trial randomized IBS-C patients to placebo or 290 μg oral linaclotide once daily in a 12-week treatment period, followed by a 4-week randomized withdrawal (RW) period. There were four primary end points, the Food and Drug Administration's (FDA's) primary end point for IBS-C (responder: improvement of ≥30% in average daily worst abdominal pain score and increase by ≥1 complete spontaneous bowel movement (CSBM) from baseline (same week) for at least 50% of weeks assessed) and three other primary end points, based on improvements in abdominal pain and CSBMs for 9/12 weeks. Adverse events (AEs) were monitored.RESULTS:The trial evaluated 800 patients (mean age=43.5 years, female=90.5%, white=76.9%). The FDA end point was met by 136/405 linaclotide-treated patients (33.6%), compared with 83/395 placebo-treated patients (21.0%) (P<0.0001) (number needed to treat: 8.0, 95% confidence interval: 5.4, 15.5). A greater percentage of linaclotide patients, compared with placebo patients, reported for at least 6/12 treatment period weeks, a reduction of ≥30% in abdominal pain (50.1 vs. 37.5%, P=0.0003) and an increase of ≥1 CSBM from baseline (48.6 vs. 29.6%, P<0.0001). A greater percentage of linaclotide patients vs. placebo patients were also responders for the other three primary end points (P<0.05). Significantly greater improvements were seen in linaclotide vs. placebo patients for all secondary end points (P<0.001). During the RW period, patients remaining on linaclotide showed sustained improvement; patients re-randomized from linaclotide to placebo showed return of symptoms, but without worsening of symptoms relative to baseline. Diarrhea, the most common AE, resulted in discontinuation of 5.7% of linaclotide and 0.3% of placebo patients.CONCLUSIONS:Linaclotide significantly improved abdominal pain and bowel symptoms associated with IBS-C for at least 12 weeks; there was no worsening of symptoms compared with baseline following cessation of linaclotide during the RW period.

Phenylketonuria (PKU) is a genetic disease that is characterized by an inability to metabolize phenylalanine (Phe), which can result in neurotoxicity. To provide a potential alternative to a protein-restricted diet, we engineered Escherichia coli Nissle to express genes encoding Phe-metabolizing enzymes in response to anoxic conditions in the mammalian gut. Administration of our synthetic strain, SYNB1618, to the Pah PKU mouse model reduced blood Phe concentration by 38% compared with the control, independent of dietary protein intake. In healthy Cynomolgus monkeys, we found that SYNB1618 inhibited increases in serum Phe after an oral Phe dietary challenge. In mice and primates, Phe was converted to trans-cinnamate by SYNB1618, quantitatively metabolized by the host to hippurate and excreted in the urine, acting as a predictive biomarker for strain activity. SYNB1618 was detectable in murine or primate feces after a single oral dose, permitting the evaluation of pharmacodynamic properties. Our results define a strategy for translation of live bacterial therapeutics to treat metabolic disorders.

We have identified an analgesic mechanism of linaclotide: it activates GC-C expressed on mucosal epithelial cells, resulting in the production and release of cGMP. This extracellular cGMP acts on and inhibits nociceptors, thereby reducing nociception. We also found that linaclotide reduces chronic abdominal pain in patients with IBS-C.

In these two 12-week trials, linaclotide significantly reduced bowel and abdominal symptoms in patients with chronic constipation. Additional studies are needed to evaluate the potential long-term risks and benefits of linaclotide in chronic constipation. (Funded by Ironwood Pharmaceuticals and Forest Research Institute; ClinicalTrials.gov numbers, NCT00765882 and NCT00730015.).

The intestine is a major source of systemic ammonia (NH3); thus, capturing part of gut NH3 may mitigate disease symptoms in conditions of hyperammonemia such as urea cycle disorders and hepatic encephalopathy. As an approach to the lowering of blood ammonia arising from the intestine, we engineered the orally delivered probiotic Escherichia coli Nissle 1917 to create strain SYNB1020 that converts NH3 to l-arginine (l-arg). We up-regulated arginine biosynthesis in SYNB1020 by deleting a negative regulator of l-arg biosynthesis and inserting a feedback-resistant l-arg biosynthetic enzyme. SYNB1020 produced l-arg and consumed NH3 in an in vitro system. SYNB1020 reduced systemic hyperammonemia, improved survival in ornithine transcarbamylase–deficient spfash mice, and decreased hyperammonemia in the thioacetamide-induced liver injury mouse model. A phase 1 clinical study was conducted including 52 male and female healthy adult volunteers. SYNB1020 was well tolerated at daily doses of up to 1.5 × 1012 colony-forming units administered for up to 14 days. A statistically significant dose-dependent increase in urinary nitrate, plasma 15N-nitrate (highest dose versus placebo, P = 0.0015), and urinary 15N-nitrate was demonstrated, indicating in vivo SYNB1020 activity. SYNB1020 concentrations reached steady state by the second day of dosing, and excreted cells were alive and metabolically active as evidenced by fecal arginine production in response to added ammonium chloride. SYNB1020 was no longer detectable in feces 2 weeks after the last dose. These results support further clinical development of SYNB1020 for hyperammonemia disorders including urea cycle disorders and hepatic encephalopathy.

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A complex interplay of metabolic and immunological mechanisms underlies many diseases that represent a substantial unmet medical need. There is an increasing appreciation of the role microbes play in human health and disease, and evidence is accumulating that a new class of live biotherapeutics comprised of engineered microbes could address specific mechanisms of disease. Using the tools of synthetic biology, nonpathogenic bacteria can be designed to sense and respond to environmental signals in order to consume harmful compounds and deliver therapeutic effectors. In this perspective, we describe considerations for the design and development of engineered live biotherapeutics to achieve regulatory and patient acceptance. Regulatory considerations for live biotherapeutic products Live biotherapeutic products (LBPs) are defined as live organisms designed and developed to treat, cure, or prevent a disease or condition in humans 10. Notably, LBPs exclude vaccines, filterable viruses, oncolytic viruses, and organisms used as vectors for transferring genes into the host. LBPs are distinguished from probiotic supplements on the basis of their labeling claims, as