Method for determining the titer of biological agents in living target cells

1. A method for determining the titer (concentration of virus or viral or non-viral vector particles) of a virus or viral or non-viral vector which interacts with living target cells and produces a product P, comprising:

(a 1 ) incubating a virus or a viral or non-viral vector at an unknown initial concentration C with living target cells at constant initial concentration D, for a time and under conditions suitable for interaction of said virus or viral or non-viral vector with the living cells, and for the production of product P, wherein measurement of a parameter of product P which reflects the response of the living target cells to the virus or viral or non-viral vector particles provides a measurable signal,

(b 1 ) measuring at various successive times t the intensity i of the signal produced by the interaction of said virus or viral or non-viral vector and said target cells,

(c 1 ) determining β, the value of i at the inflexion point of the i=f(t) curve for the virus or viral or non-viral vector,

(c 2 ) determining the time t β corresponding to the i=β value, chosen in the interval β min <β<β max , wherein β min and β max , respectively correspond to the values of i at the inflexion point of the i=f(t) curve for the minimal and maximal concentrations of a standard virus or viral or non-viral vector, for which the t β =f(C) curve is pre established, and

(d 1 ) determining the initial concentration C of the virus or viral or non-viral vector using a t β =f(C) standard curve.

2. The method of claim 1 , wherein, simultaneously or before the step (a 1 ), the standard curve is established according to the following steps:

(a 0 ) preparing n serial dilutions of a standard virus or viral or non-viral vector at a known initial concentration C 0 , corresponding to said virus or viral or non-viral vector final concentrations C 1 , C 2 , . . . C n , respectively,

(b 0 ) incubating each standard virus or viral or non-viral vector dilution obtained in (a 0 ), with said target cells at a constant initial concentration D,

(c 0 ) determining the intensity i of the signal produced by the interaction of the virus or viral or non-viral vector and the living target cells for each standard virus or viral or non-viral vector concentration C 1 , C 2 , . . . C n ′ at successive times t 1 to t n ′

(d 0 ) drawing the i=f(t) curve for each value of C 1 , C 2 , . . . C n ′

(e 0 ) determining the 13 value of i, chosen in the interval β min <β<β max wherein β min and β max respectively correspond to the values of i at the inflexion point of the i=f(t) curve for C n , and C 1 , and

(f 0 ) drawing the t β =f(C) standard curve for the i=β value.

3. The method of claim 1 , wherein the virus or viral or non-viral vector is a virus.

4. The method of claim 1 , wherein the virus or viral or non-viral vector is a virus gene transfer vector.

5. The method of claim 1 , wherein the virus or viral or non-viral vector is a non-viral gene transfer vector.

6. The method of claim 1 , wherein the biological agent is a viral vaccine.

7. The method of claim 1 , wherein the measurable signal is obtained by measuring a parameter of the virus or viral vector genome copy number in said target cells.