Table 1. Proteasome inhibitors.

Compound/Origin (if natural)	Activity, IC50, nM			Type of assay	Ref.
	CT-L	T-L	C-L
Peptide aldehydes
Felutamide B (Penicillium fellutanum)	9.4	2000	1200	20S	[65]
TP-110	27			20S	[66]
MG132	68	4500	1400	20S	[66]
Tyropeptin A (Kitasabospora sp.)	40			20S	[65]
	1400	5000	68 000	20S	[66]
	Dose-dependent			PC12 cells	[67]
Syrbactins
Glidobactin A (Burkholderiales)	15	>15		20S	[68]
Syringolin A (Pseudomona syringae)	1300	>1300	>1300	20S	[68]
Epoxyketones
Epoxomicin (Actinomycetes)	5.7			20S	[65]
NC-022		300		20S	[69]
NC-001			500	MM1.R, NCI-H929	[70]
YU-102			+	20S	[71]
Vinyl esters
HMB-LLL-VE	41	4210	>10 000	20S	[72]
Peptide Vinyl Sulfones
MB1		+		20S	[73]
Natural polyphenols
Epigallocatechin gallate / EGCG (tea polyphenols)	86–194			20S	[74]
	1000–10000			Living Jurkat T cells	[75]
Curcumin / diferuloylmethane (Curcuma longa)	1850	6230	3680	20S	[76]
	20000			HCT-116 and SW480 cells	[76]
Natural cationic porphyrins
H2T4	750	530	460	20S and 26S	[77]
β-lactones
Omuralide / clasto-lactacystin-β-lactone (Salinospora tropica)	29	690	8300	20S	[78]
	600			Living Jurkat T cells	[75]
Hydronaphthoquinones
PI-083	1000	4500	4500	20S	[79]
	> 1000	> 4500	> 4500	Nude mice	[79]
Isothiocyanates
BITC	4700	~10000	~10000	A549 cells	[80]
Triterpenoids
Pristimerin (Celastrus Maytenus)	<100			20S	[81]
	50 000			H929 and U266 U266 cells	[81]
Celastrol (Tripterygium Wilfordi, Celastrus Regelii)	2500			A6 Xenopus cells, mammalian cells, human prostate cancer cells	[82]
	1000–5000			Nude mice PC-3 or LNCaP cells	[83]
Chalcones
AM114	1000			20S	[84]
	1490			HCT116 p53 +/+ cells	[84]
Imidazoline derivatives
TCH-013	2800		1600	20S	[85]
Carbamides and carbamates
Hydroxyurea	1000			20S	[86]
DSF / disulfiram	2000			MDA-MB-231 cells	[87]
DSF Cu (II)	7500			20S	[88]
	>7500			26S	[88]
DSF Cd	3500			20S	[88]
	3200			hMCF 10 DCIS cells and PC-3 cells	[87]
Inorganics
CuCl2	5100			20S	[89]

A summary of proteasome inhibitors known to-date grouped by: their origin (synthetic or natural), targeting specificity and IC50 against the proteasomal enzymatic activities (CT-L stands for ChymoTrypsin-Like, T-L stands for Trypsin-Like and C-L stands for Caspase-Like activity), and type of assay used for measuring the inhibitory effect. The compounds are also divided into classes according to their chemical structures (aldehydes, syrbactins, epoxyketones, etc.).