Discussion
With a sophisticated and comprehensive, multimodal PK-PD, safety
approach we investigated a possibly minimally invasive administration
method of adalimumab with a commercially available hollow microneedle.
Importantly, this clinical trial shows that i.d. administration of a
single dose of 40 mg adalimumab in a volume of 0.4 mL using a hollow
microneedle is safe and well accepted. However, i.d. administration was
associated with an increased amount of injection pain and decreased
volunteer preference compared to s.c. administration. Using imaging
methods, the effect of i.d. injections on the skin was thoroughly
characterized. As expected, i.d. injections led to bleb formation.
Notably, i.d. injection transiently increased cutaneous microcirculation
as measured by LSCI. Importantly, we found that i.d. administration of
adalimumab led to a higher Cmax and a higher
bioavailability compared to s.c. adalimumab administration. The
inhibition of ex vivo cytokine production of whole blood
stimulated with LPS/Alum was similar for i.d. and s.c. adalimumab
administration indicating comparable pharmacodynamic efficacy.
Protein degradation, especially aggregation, might result in increased
immunogenicity of mAbs (11) and immunogenicity of mAbs is a major reason
for secondary loss of response to mAbs. Therefore, we first showedin vitro that microneedle ejection of adalimumab does not
substantially alters the amount of protein fragments or aggregates
compared to ejection using a regular hypodermic needle.
Hollow microneedles are frequently considered a minimally invasive
device to deliver parenteral drugs (4,25–28). In this study we
administered a single adalimumab dose of 40 mg in 0.4 mL or 0.4 mL
placebo. We systematically studied pain associated with insertion and
injection in a double-blind manner. We found that insertion pain of s.c.
and i.d. administration was equal. However, injection pain of i.d.
administration was significantly higher than s.c. administration. The
high amount of pain is in contrast with another study, which used higher
volumes but detected less pain (28). Pain due to s.c. injection is
generally attributed to different factors, i.e., volume of injection,
site of injection, formulation, needle size, and injection depth (29).
The volume limit of s.c. injection is generally considered to be 1.5 mL
(30). Several studies have found higher volumes of s.c. administration
to be associated with more pain (30–32). Thus, the increased pain that
was associated with i.d. administration in the clinical trial reported
in this paper is likely due to the volume injected. The volume used in
this trial was limited by a minimum volume which contains a regular dose
of a mAb in adults. Future studies might investigate the volume-pain
relationship for i.d. administration using hollow microneedles. We did
not detect a significant difference in pain when comparing adalimumab
with placebo after i.d. and s.c. administration, which indicates that
the formulation chosen in this study did not influence pain.
Although not quantified, we observed a higher injection pressure during
i.d. administration compared to s.c. administration. With OCT, we
detected fluid filled cavities after i.d. injection, indicating there
was no time for the compound to distribute in the skin.
We characterized the skin response to hollow microneedle administration
of adalimumab using a combination of methods. The skin response
following i.d. administration of adalimumab was mild and resolved within
a day after injection. Using 3D photography, we showed the bleb which is
typical for i.d. administration. Furthermore, using LSCI, an increase in
cutaneous microcirculation after i.d. injection of adalimumab was
observed. Our observations are of interest in the context of drug
absorption. The increased cutaneous microcirculation might be associated
with the increased adalimumab absorption following i.d. versus s.c.
administration observed in our study. Yet, drugs injected s.c. may be
absorbed via the lymph capillaries, or diffuse into blood capillaries,
and after s.c. administration proteins with a high molecular weight,
such as mAbs, are predominantly absorbed via the lymph after s.c.
administration (33,34).
Various factors influence lymph flow, one being local skin temperature.
During an increase in local skin temperature, both the blood flow and
the lymph flow increase (35–37). We quantified local skin temperature
after i.d. adalimumab administration using thermography. A limitation is
that from the skin temperature measurements we cannot unequivocally
conclude which type of injection (s.c. or i.d.) leads to higher skin
temperature for two reasons. The temperature measurements might be
confounded by difference in depth as i.d. injections are more
superficial than s.c. injections. Thus, the s.c. injections might have
increased the local temperature which is not apparent from our
measurements.
Initial lymphatics, the part of the lymph vessels responsible for drug
uptake, are located superficially, in the dermis (38). Under
physiological conditions most of these lymph vessels are collapsed.
Excess fluid (high hydrostatic pressure) and proteins (high local
osmotic pressure) in the dermis cause high lymph flow. We used OCT to
visualize epidermal penetration after i.d. injection. Qualitative
analysis of OCT observations showed an increase in vessel diameter after
i.d. injection compared to s.c. injection. Based on the OCT, no
distinction can be made between blood and lymph vessels. Perhaps in the
future a new variant of OCT, Doppler OCT (39), could be used to further
characterize the physiology of mAb absorption and lymph flow.
Several studies have reported that the i.d. administration of drugs has
different PK characteristics than s.c. delivery (5,7,28,40). General
observations are that Tmax is decreased,
Cmax is increased and that bioavailability is either
equal or increased after i.d. administration compared to s.c.
administration. Most studies use insulin as model drug. For i.d.
injection of insulin using hollow microneedles, it has been reported
that Cmax increases and Tmax decreases
after i.d. administration versus s.c. administration. It has been
suggested that a shift in the concentration-time profile explains why
some but not all studies have reported increased bioavailability after
i.d. injection (5,41). Changes in PK are generally attributed to
anatomical differences in the skin: the dermis has extensive vasculature
and lymphatics while the subcutis has more adipose tissue (42). When
correcting for individual differences in the covariates and the titre
values, this study showed a significant difference in bioavailability
between s.c. and i.d. administration; i.d. administration was associated
with a 29% higher bioavailability. In our study, a clear distinction in
the absorption profiles over time could be observed between s.c. and
i.d. administration. Adalimumab administered by microneedle injection
show a short but fast absorption, whereas s.c. dosing shows a lower
absorption rate. The steep drop in absorption after a microneedle
injection is caused by the distribution of sampling points and an
estimated mathematical time point. In reality, this transition would
probably be smoother. Altogether, the PK profile of the i.d.
administration of adalimumab is favourable over s.c. administration.
The immunogenicity of mAbs is a significant clinical problem hampering
the treatment of autoimmune diseases with mAbs. In this study the number
of healthy volunteers allows only for descriptive reporting of
anti-adalimumab antibodies. The skin is a potent immune organ (42).
Studies have shown an increased immunogenicity of i.d. vaccines compared
to s.c. vaccines and microneedles are frequently studied as a device to
deliver vaccines (43,44). On the other hand, it has been suggested that
i.d. administration of mAbs might lead to less immunogenicity compared
to s.c. administration due to the presence of professional antigen
presenting cells in the epidermis and dermis rather than in the subcutis
(33,45). Perhaps the relatively short residence time at the i.d.
injection site of the (predominantly monomeric) protein might contribute
to the lack of increased immunogenicity as compared to s.c.
administration. It remains to be determined whether i.d. administration
of biologicals alters the incidence, degree, or time of onset of
anti-drug antibody formation compared to s.c. administration.
In this study the functional effect of adalimumab administration was
investigated in vitro . Whole blood was stimulated with LPS/Alum
and secreted cytokines were measured. We found that i.d. and s.c.
adalimumab reduced ex vivo TNFα bioavailability to a similar
extent.
The increased bioavailability of i.d. adalimumab in our study suggests
that lower doses may be used to achieve similar concentrations and
subsequent effects compared to s.c. administration. Combined with the
increased elasticity of the skin of children (46) and the need for a
lower (adalimumab) dose than in adults, hollow microneedles ultimately
might be suitable for use in paediatric patients. However, it is of
paramount importance to better understand the pain-volume relationship
of i.d. injections using hollow microneedles in adults first.
In conclusion, we showed that the i.d. administration of adalimumab is
feasible and leads to faster absorption and increased bioavailability
compared to s.c. administration. The amount of pain reported in this
study, higher for i.d. than for s.c. adalimumab administration, is
likely explained by the injection volume of 0.4 mL. Understanding the
relationship between pain and the administration of mAbs is essential
before hollow microneedles can be investigated for use in the paediatric
patient population.