TABLE 2.

Association and dissociation rates and affinities of actoxumab and bezlotoxumab binding to purified toxins, as determined by surface plasmon resonance

Antibody/toxin	Strain	High-affinity binding site			Low-affinity binding site
		kon (M−1 s−1)a	koff (s−1)	K′d (nM)	kon (M−1 s−1)	koff (s−1)	K′d (nM)
Actoxumab/TcdA	087b	1.9 × 105 ± 0.1 × 105	1.1 × 10−4 ± 0.1 × 10−4	0.61 ± 0.09	NDc	ND	ND
	027	1.7 × 104 ± 0.2 × 104	1.5 × 10−4 ± 0.2 × 10−4	8.7 ± 1.5	ND	ND	ND
	078	5.0 × 104 ± 3.4 × 104	2.2 × 10−4 ± 1.0 × 10−4	5.2 ± 2.2	ND	ND	ND
Bezlotoxumab/TcdB	087	1.0 × 106 ± 0.1 × 106	5.2 × 10−5 ± 2.0 × 10−5	0.050 ± 0.014	4.8 × 106 ± 1.4 × 106	1.6 × 10−2 ± 0.8 × 10−2	3.9 ± 2.9
	027	3.9 × 105 ± 0.8 × 105	2.8 × 10−4 ± 0.3 × 10−4	0.75 ± 0.16	ND	ND	ND
	078	6.7 × 105 ± 0.3 × 105	4.4 × 10−4 ± 1.9 × 10−4	0.65 ± 0.27	ND	ND	ND

^aValues are means ± standard deviations of two separate determinations.

^bStrain 087 is VPI 10463.

^cND, not determined because the data best fit a single-site model.