Bortezomib is the therapeutic proteasome inhibitor. First, which is tested in humans. The boron atom in bortezomib binds the catalytic site of the 26S proteasome with high affinity and specificity. Bortezomib is approved in the U.S. for treating relapsed multiple myeloma and mantle cell lymphoma. The 26S proteasome degrades various proteins critical to cancer cell survival, such as cyclins, tumor suppressors, BCL-2, and cyclin-dependent kinase inhibitors. Inhibition of these degradations sensitizes cells to apoptosis. Bortezomib is a potent inhibitor of 26S proteasome, which sensitizes activity in dividing multiple myeloma and leukemic cells, thus inducing apoptosis. Most commonly reported adverse reactions (incidence ≥30%) in clinical studies include asthenic conditions, diarrhea, nausea, constipation, peripheral neuropathy, vomiting, pyrexia, thrombocytopenia, psychiatric disorders, anorexia and decreased appetite, neutropenia, neuralgia, leukopenia and anemia. Co-administration of ketoconazole, a potent CYP3A inhibitor, increased the exposure of bortezomib. Co-administration of melphalan-prednisone increased the exposure of bortezomib. However, this increase is unlikely to be clinically relevant.

Tadalafil is used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH). Part of the physiological process of erection involves the release of nitric oxide (NO) in the corpus cavernosum. This then activates the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation in the corpus cavernosum, resulting in increased inflow of blood and an erection. Tadalafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. This means that, with tadalafil on board, normal sexual stimulation leads to increased levels of cGMP in the corpus cavernosum which leads to better erections. Without sexual stimulation and no activation of the NO/cGMP system, tadalafil should not cause an erection.Tadalafil inhibits the cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum located around the penis. Penile erection during sexual stimulation is caused by increased penile blood flow resulting from the relaxation of penile arteries and corpus cavernosal smooth muscle. This response is mediated by the release of nitric oxide (NO) from nerve terminals and endothelial cells, which stimulates the synthesis of cGMP in smooth muscle cells. Cyclic GMP causes smooth muscle relaxation and increased blood flow into the corpus cavernosum. The inhibition of phosphodiesterase type 5 (PDE5) by tadalafil enhances erectile function by increasing the amount of cGMP. Tadalafil is used for the treatment of erectile dysfunction.

Miglustat, an N-alkylated imino sugar, is a synthetic analogue of D-glucose. Miglustat is an inhibitor of the enzyme glucosylceramide synthase, which is a glucosyl transferase enzyme responsible for catalyzing the formation of glucosylceramide (glucocerebroside). Glucosylceramide is a substrate for the endogenous glucocerebrosidase, an enzyme that is deficient in Gaucher's disease. The accumulation of glucosylceramide due to the absence of glucocerebrosidase results in the storage of this material in the lysosomes of tissue macrophages, leading to widespread pathology due to infiltration of lipid-engorged macrophages in the viscera, lymph nodes, and bone marrow. This results in secondary hematologic consequences including sever anemia and thrombocytopenia, in addition to the characteristic progressive hepatosplenomegaly, as well as skeletal complications including osteonecrosis and osteopenia with secondary pathological fractures. Miglustat functions as a competitive and reversible inhibitor of the enzyme glucosylceramide synthase, the initial enzyme in a series of reactions which results in the synthesis of most glycosphingolipids. The goal of treatment with miglustat is to reduce the rate of glycosphingolipid biosynthesis so that the amount of glycosphingolipid substrate is reduced to a level which allows the residual activity of the deficient glucocerebrosidase enzyme to be more effective (substrate reduction therapy), reducing the accumulation of glucocerebroside in macrophages. In vitro and in vivo studies have shown that miglustat can reduce the synthesis of glucosylceramide-based glycosphingolipids. In clinical trials, miglustat improved liver and spleen volume, as well as hemoglobin concentration and platelet count. Inhibition of glycosphingolipid synthesis has also shown to reduce intracellular lipid storage, improve fluid-phase endosomal uptake and normalize lipid transport in peripheral blood B lymphocytes of NP-C patients, which results in a decrease in the potentially neurotoxic accumulation of gnagliosides GM2 and GM3, lactosylceramide and glucosylceramide, possibly preventing further neuronal damage. Other studies have also suggested that miglustat may indirectly modulate intracellular calcium homeostasis through its effects on glucosylceramide levels, and evidence has shown that an initiating factor in the pathogenesis of NP-C may be impaired calcium homeostasis related to sphingosine storage. Therefore, the effect that miglustat exerts on intracellular calcium levels may influence an important underlying pathogenic mechanism of NP-C. Miglustat is used for the treatment of adult patients with mild to moderate type 1 (nonneuropathic) Gaucher's disease for whom enzyme replacement therapy is not a therapeutic option (e.g. due to constraints such as allergy, hypersensitivity, or poor venous access). Now approved in some countries for the treatment of progressive neurological symptoms in adult and pediatric patients with Niemann-Pick disease type C (NP-C). Miglustat is marketed under the trade name Zavesca.

Nitisinone, 2-(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) is a triketone with herbicidal activity. Orfadin® capsules contain nitisinone used in the treatment of hereditary tyrosinemia type 1 (HT-1). Nitisinone is a competitive inhibitor of 4-hydroxyphenyl-pyruvate dioxygenase, an enzyme upstream of fumarylacetoacetase in the tyrosine catabolic pathway. By inhibiting the normal catabolism of tyrosine in patients with HT-1, nitisinone prevents the accumulation of the catabolic intermediates maleylacetoacetate and fumarylacetoacetate. In patients with HT-1, these catabolic intermediates are converted to the toxic metabolites succinylacetone and succinylacetoacetate, which are responsible for the observed liver and kidney toxicity. Succinylacetone can also inhibit the porphyrin synthesis pathway leading to the accumulation of 5-aminolevulinate, a neurotoxin responsible for the porphyric crises characteristic of HT-1. Zeneca Agrochemicals and Zeneca Pharmaceuticals made NTBC available for clinical use and, with the approval of the Swedish Medical Products Agency, a seriously ill child with an acute form of tyrosinaemia type 1 was successfully treated in February 1991. Nitisinone is investigated as a potential treatment for other disorders of tyrosine metabolism including alkaptonuria.

Galantamine (RAZADYNE®, galantamine hydrobromide) is a benzazepine derived from norbelladine. It is found in Galanthus and other Amaryllidaceae. It is a reversible, competitive acetylcholinesterase inhibitor that is used for the treatment of mild to moderate dementia of the Alzheimer’s type. Although the etiology of cognitive impairment in Alzheimer’s disease is not fully understood, it has been reported that acetylcholine-producing neurons degenerate in the brains of patients with Alzheimer’s disease. The degree of this cholinergic loss has been correlated with degree of cognitive impairment and density of amyloid plaques (a neuropathological hallmark of Alzheimer’s disease). While the precise mechanism of galantamine’s (RAZADYNE®, galantamine hydrobromide) action is unknown, it is postulated to exert its therapeutic effect by enhancing cholinergic function. This is accomplished by increasing the concentration of acetylcholine through reversible inhibition of its hydrolysis by acetylcholinesterase. If this mechanism is correct, galantamine’s (RAZADYNE®, galantamine hydrobromide) effect may lessen as the disease process advances and fewer cholinergic neurons remain functionally intact. There is no evidence that galantamine (RAZADYNE®, galantamine hydrobromide) alters the course of the underlying dementing process.

Rivastigmine (sold under the trade name Exelon) is a parasympathomimetic or cholinergic agent for the treatment of mild to moderate dementia of the Alzheimer's type and dementia due to Parkinson's disease. Rivastigmine, an acetylcholinesterase inhibitor, inhibits both butyrylcholinesterase and acetylcholinesterase (unlike donepezil, which selectively inhibits acetylcholinesterase). It is thought to work by inhibiting these cholinesterase enzymes, which would otherwise break down the brain neurotransmitter acetylcholine. Rivastigmine capsules, liquid solution, and patches are used for the treatment of mild to moderate dementia of the Alzheimer's type and for mild to moderate dementia related to Parkinson's disease. Rivastigmine has demonstrated treatment effects on the cognitive (thinking and memory), functional (activities of daily living) and behavioral problems commonly associated with Alzheimer's and Parkinson's disease dementia. In people with either type of dementia, rivastigmine has been shown to provide meaningful symptomatic effects that may allow patients to remain independent and ‘be themselves’ for longer. In particular, it appears to show marked treatment effects in patients showing a more aggressive course of the disease, such as those with younger-onset ages, poor nutritional status, or those experiencing symptoms such as delusions or hallucinations. Side effects may include nausea and vomiting, decreased appetite and weight loss.

Orlistat or tetrahydrolipstatin (Xenical, Hoffmann-La Roche) is a saturated derivative of lipstatin originally isolated from Streptomyces toxytricini. Orlistat (Xenical, Hoffmann-La Roche) is a powerful inhibitor of gastrointestinal lipase and as such, reduces fat absorption. Orlistat acts by binding covalently to the serine residue of the active site of gastric and pancreatic lipases. When administered with fat-containing foods, orlistat partially inhibits hydrolysis of triglycerides, thus reducing the subsequent absorption of monoaclglycerides and free fatty acids. Unlike other weight-reducing drugs it is minimally absorbed and has no effects in the CNS. Xenical is indicated for obesity management including weight loss and weight maintenance when used in conjunction witha reduced-calorie diet. XENICAL is also indicated to reduce the risk for weight regain after prior weight loss. XENICAL is indicated for obese patients with an initial body mass index (BMI) ≥ 30 kg/m2 or ≥ 27 kg/m2 in the presence of other risk factors (eg, hypertension, diabetes, dyslipidemia). In addition to its well established efficacy in achieving modest weight loss, orlistat has been shown to improve glycaemic parameters in obese adults with type 2 diabetes mellitus as well as some features of the metabolic syndrome.

Cilostazol is a PDE3 inhibitor which is used for the treatment of intermittent claudication. The drug positively affects the platelet aggregation and may be used off-label as a measure to prevent coronary thrombosis/restenosis and stroke recurrence.

Sildenafil (Viagra, Revatio) is a PDE5 inhibitor which was approved by FDA for the treatment of erectile disfunction and adults with pulmonary arterial hypertension. Upon administration sildenafil inhibits PDE5 and results in elevated level of cyclic guanosine monophosphate and smooth muscle relaxation.

Teriflunomide (trade name Aubagio, marketed by Sanofi) is the active metabolite of leflunomide and it acts as an immunomodulatory agent by inhibiting pyrimidine synthesis by blocking the enzyme dihydroorotate dehydrogenase. Teriflunomide was investigated in the Phase III clinical trial TEMSO as a medication for multiple sclerosis (MS). The drug was approved by the FDA on September 13, 2012 and in the European Union on August 26, 2013. It is uncertain whether this explains its effect on MS lesions. Teriflunomide inhibits rapidly dividing cells, including activated T cells, which are thought to drive the disease process in MS. Teriflunomide may decrease the risk of infections compared to chemotherapy-like drugs because of its more-limited effects on the immune system. It has been found that teriflunomide blocks the transcription factor NF-κB. It also inhibits tyrosine kinase enzymes, but only in high doses not clinically used.