CRIM Status, Genotype, Residual Enzyme Activity and Immunogenicity in ERTs

Immune response associated adverse events and hypersensitivity reactions are the major safety concerns related to enzyme replacement therapies. The immunogenicity against these therapeutic enzymes depends on structural similarities, manufacturing processes, formulation, and impurities etc. The drug developer has developed strategies to mitigate manufacturing related immunogenicity risk with improved processes such as optimization of the protein sequence, mammalian cell lines to express and purify recombinant human enzymes. Despite these efforts, the immunogenicity and anti-drug antibody generation against these therapeutic proteins are heterogeneous among the populations. In the same dose cohort, one subject may produce higher anti-drug antibodies titer with lesser efficacy. In contrast, another subject may have less or no anti-drug antibodies with no impact on drug efficacy. Identify and characterizing these individual factors would enable a better understanding of individual factors causal for these heterogeneities.

Among these individual factors, the residual enzyme activity, cross-reactive immunological material (CRIM) status and genotype are being characterized in each subject to determine if any of these factors correlate with immunogenicity and safety efficacy.

Measurement of Residual enzyme activity
For genetic disorders of enzyme deficiencies, enzyme activity is measured in the clinically suspected patients and diagnosis are made based on the decreased enzyme activity. The enzyme activity in affected subjects is usually less than 5-10% (residual enzyme activity) of the normal population with an accumulation of substrate to a toxic level in various tissues. The loss of enzyme activity may be the result of any genetic mutation such as missense mutations, frame-shift mutations, a mutation that allows alternative splicing, or nonsense mutation leading to some or no protein expression. The residual enzyme activity may correlate with phenotype and severity of the disease in the population but do not predict the presence/absence of enzymes.

Detection of Cross-reactive immunological material (CRIM)
Immunogenicity against therapeutic proteins may be associated with the residual enzyme/protein in the affected subjects. The presence of residual proteins (enzyme) may induce the immune tolerance and the exogenous proteins may be recognized as self-protein. Therefore, it may have a lower tendency to induce an immune response against these individuals. In contrast, the individual that have undetectable residual protein may higher probability to induce an immune response against the therapeutic protein. To test this hypothesis, multiple studies have been conducted to evaluate the association of CRIM with patients’ immune response. The studies showed variable outcomes with positive correlations to no correlations of immunogenicity to CRIM status suggesting an interplay of additional genetic and immunological factors in immunogenicity and treatment outcome.

Characterization of underlying genetic mutations:
Although the genetic mutation in a causal gene leads to the specific disease, the heterogeneity in disease phenotype and severity may be associated with the mutation type. Similarly, the propensity of inducing immune response may also be associated with the underlying mutation. Therefore, characterization of the patient genotype and correlation with the disease phenotype, immunogenicity, and treatment outcome may provide clues on the variable outcome for safety and efficacy in each subject. To understand this, numerous studies are carried out to correlate if the patient genotype is associated with immunogenicity and disease outcome.

Summary and Conclusion:
The studies have shown that CRIM status and patient genotype are partially associated with the immunogenicity, and safety/efficacy and treatment outcome. A recent publication on the rare genomic variant and phenotype correlation have shown the association and predictive value of the genomic mutation on disease outcomes for the various studied monogenic disease. Predicting the disease outcome based on these factors require more extensive genomic association studies and a larger study population.