Cell Transplantation as an Experimental Treatment for Duchenne Muscular Dystrophy

Abstract

The feasibility, safety, and efficacy of myoblast transfer therapy (MTT) were assessed in an experimental lower body treatment (LBT) involving 32 Duchenne muscular dystrophy (DMD) boys aged 6-14 yr, half of whom were nonambulatory. Through 48 injections, five billion (55.6 × 106/ mL) normal myoblasts were transferred into 22 major muscles in both lower limbs, in 10 min with the subject under general anesthesia. Ten subjects received myoblasts cultured from satellite cells derived from 1-g fresh muscle biopsies of normal males aged 9-21 yr. Donor myoblasts for the remaining 22 boys were subcultured from reserves frozen 1 mo-1.5 yr ago. Only four donors were known to have identical histocompatibility with their recipients. All subjects took oral doses of the immunosuppressant cyclosporine (Cy), beginning at 2 days before MTT and lasting for 6 mo after MTT to facilitate donor cell survival. There was no evidence of an adverse reaction to MTT or Cy as determined by serial laboratory evaluations including electrolytes, creatinine, and urea. Objective functional tests using the KinCom Robotic Dynamometer measured the maximum isometric contractile forces of the ankle plantar flexors (AF), knee flexors (KF), and knee extensors (KE) before MTT and at 3, 6, and 9 mo after MTT. The AF, being distal muscles and less degenerative than the KE and the KF, showed no decrease in mean contractile force 3 mo after MTT, and progressive increases in force at 6 and 9 mo after MTT. At 9 mo after MTT, 60% of the 60 AF examined showed a mean increase of 50% in force; 28% showed no change; and only 12% showed a mean decrease in force of 29% when compared to the function of the same muscles before MTT. The KF, being proximal muscles and more degenerative, showed no change in function at 9 mo after MTT. The KE, being proximal and anti-gravitational, were most degenerative before MTT. They showed no statistically significant change in force at 3 mo after MTT but showed decreases at 6 and 9 mo after MTT. At 9 mo after MTT, 23% of the 60 KE examined showed a mean increase of 65% in force; 22% showed no change; and 55% showed a mean decrease of 24% in force. When results of all muscle groups (AF, KF, KE) were pooled, there was no change in force at 3, 6, or 9 mo after MTT vs. before MTT according to the Wilcoxon signed rank test. The ambulatory subjects showed more muscle improvement than the nonambulatory ones at various times after MTT. Statistically significant progressive increase in force in the AF and arrest of weakening in the KF and KE were observed in the ambulatory subjects as early as 3 mo and continued up to 9 mo after MTT. Statistically significant changes were confirmed with analyses of variance. The results indicate that (a) MTT is safe; (b) MTT improves muscle function in DMD: 88% of the AF, 49% of the KF, and 45% of the KE showed either increase in strength or did not show continuous loss of strength 9 mo after MTT; (c) the dosage used is more effective in the AF than in the KF, and is least effective in the KE; (d) more than 5 billion myoblasts are necessary to strengthen both lower limbs of a DMD boy between 6 and 14 yr of age; (e) the more degenerated proximal muscles will need more myoblasts per unit muscle volume than the distal muscles for MTT to be effective; (f) MTT is more effective in the younger, ambulatory subjects than in the older, nonambulatory subjects; (g) Cy is not responsible for the functional improvement, because muscle function continues to improve 9 mo after MTT despite Cy withdrawal at 6 mo after MTT; (h) Cy immunosuppression permits donor cell survival and development, without overt rejection symptoms, when properly managed; (i) myoblasts from frozen reserves are as effective as those from fresh muscle biopsies; (j) fifteen billion myoblasts can be cultured from a 1-g muscle biopsy; (k) billions of cultured myoblasts can be injected into subjects without tumor formation.