 |
Description  |
|
|
BACKGROUND OF THE INVENTION
Atherosclerosis is a disease wherein fatty substances (lipids), hereinafter
referred to as atheromatous plaques, form deposits in and beneath the
intima which is the innermost tissue lining arteries and veins. Clinical
symptoms occur because the mass of the atherosclerotic plaque reduces
blood flow through the involved artery, comprising tissue or organ
function.
In the co-pending patent application Ser. No. 443,958 now U.S. Pat. No.
4,512,762, a treatment for artherosclerosis is disclosed. This treatment
is based on the discovery that hematoporphyrin derivative (HPD)
selectively accumulates in atheromatous plaques and not in the adjacent
normal tissue. These plaques can be treated or removed by photoactivating
the porphyrin(s) with visible light, which apparently causes the release
of singlet oxygen which in turn damages the atheromatous plaque cell. This
cytotoxic mechanism is discussed in "Identification of Singlet Oxygen As
The Cytotoxic Agent in Photoinactivation of A Murine Tumor." by K. R.
Weishaupt, C. J. Gromer, and T. J. Dougherty, Cancer Research,
36:2326-2329 (1976).
Photoactivation of the HPD, as taught by patent application Ser. No.
443,958 now U.S. Pat. No. 4,512,762, can be accomplished by either of two
different techniques. With one technique, the patient is catheterized with
a light-emitting catheter inserted into the diseased artery until the
light-emitting portion of the catheter is adjacent the atheromatous
plaque. With the alternative technique, light-emitting liquid, such as the
aqueous peroxyalate chemiluminescent system made by American Cyanamid Co.,
is injected into the vascular tree so that the liquid, which mixes freely
with the blood or a blood replacement, perfuses the diseased artery and
photoactivates the absorbed hematoporphyrin.
The referenced application teaches an invasive means for reducing or
removing atheromatous plaques. No conventional invasive test is currently
available which can be used to reliably image or quantitate atheromatous
plaques. Arteriography is a definitive procedure for determining the
extent of human encroachment by plaques, but cannot be used to image the
plaques per se. During a catherization procedure, an ultrathin fiberoptic
catheter (an angioscope) is introduced into the arterial tree. The lumen
may be visualized directly by displacing the blood with an optically clear
medium such as one of the presently available perfluorocarbon-containing
blood replacements which provide both oxygen transport and maintain
oncotic pressure. Demarcation between the artheromatous plaque and the
normal artery wall is ambiguous, however, because the luminal surface of
both tissues is the same whitish color. A technique for direct
visualization of the blood vessel walls is disclosed in "In Vivo Coronary
Angioscopy" by J. Richard Spears, H. John Marais, Juan Serur, Oleg
Pomerantzeff, Robert P. Geyer, Robert S. Sipzener, Ronald Weintraub,
Robert Thurer, Sven Paulin, Richard Gerstin and William Grossman, J. Amer.
College of Cardiology, 1(5):1311-1314(1983).
Noninvasive techniques for detecting or defining atheromatous plaques are
also quite limited. Ultrasound can be used to detect lumen encroachment by
atheromatous plaques in the carotid artery, fluoroscopy can be used to
identify plaques if they contain calcium deposits, and NMR can
occasionally be used to identify plaques. None of these is a very reliable
means for the noninvasive imaging of atheromatous plaques. As a result,
currently used non-invasive tests for atherosclerotic coronary artery
disease are based solely on the physiological consequences of lumen
encroachment by the atheromatous plaques. Such tests include ECG, ECG
stress testing, thallium perfusion imaging, and radionuclide
ventriculography. A severe stenosis (approximately 70% diameter reduction)
must be present, however, before an abnormality is detected by these
tests.
The primary reason that atheromatous plaques cannot be reliably detected by
the use of currently available noninvasive techniques is that atheromatous
plaques do not differ significantly from surrounding tissues in any of the
physio-chemical properties which alter the signals detected by the
techniques.
It is therefore an objective of the present invention to provide a means
for detecting, imaging, or quantifying atheromatous plaques which can be
used with both invasive and noninvasive techniques.
SUMMARY OF THE INVENTION
In accordance with the present invention, the detection of the selective
absorbtion of porphyrin(s) by atheromatous plaques can be used to image,
localize, or quantify atheromatous plaques. The porphyrin(s) generate a
signal which differentiates the plaque from the adjacent normal tissue.
Various types of signals are possible. Chemically unaltered porphyrins
fluoresce when exposed to ultraviolet light and the fluorescence may be
detected using invasive means at the time of surgery, postmortem
examination, or during a catheterization procedure. Chemically altered
porphyrins, absorbed by atheromatous plaques, may be detected using
non-invasive techniques. Radiolabeled porphyrins may be detected using
radionuclide scintographic techniques. Porphyrins labelled with
short-lived tracers may be detected by Positron Emission Tomography (PET).
Porphyrins labelled with radio-opaque markers may be used to visualize
atheromatous plaques fluoroscopically. Porphyrins incorporating
short-lived isotopes within their structures can be detected by nuclear
magnetic resurance (NMR). Other means may be used where the presence of
the porphyrin increases the measured signal-to-noise ratio between normal
and diseased tissues.
The absorbtion of these porphyrins by the atheromatous plaques may be
increased and their detection enhanced by binding antibody specific for
components of the plaques to the porphyrins.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In one embodiment of the present invention, a solution of porphyrins is
injected into a peripheral vein from which it disseminates throughout the
vascular system. The dosage depends on the purity and chemical form of the
porphyrin and the degree of absorbtion expected. The most commonly used
porphyrin is hematoporphyrin, which may be in purified form or mixed with
other porphyrins. Other porphyrins not present in HPD, such as
uroporphyrin I dihydrochloride, which may selectively localize within
atheromatous plaques, may be used. For the purposes of this invention,
"porphyrins" is defined as a solution containing one or more of these
porphyrins.
When using hematoporphyrin derivative (HPD), available from Oncology
Research and Development, as described in the copending patent application
Ser. No. 443,958, the usual dosage is between 2.5 mg hematoporphyrin/kg
body weight and 10 mg hematoporphyrin/kg body weight.
Other more highly purified hematoporphyrin preparations are now
commercially available. These include Dihematoporphyrin ether (DHE) and
Photofrin II, produced by Photofrin Medical, Inc.
The injected porphyrin is selectively absorbed into the atheromatous
plaques, with the peak concentration occurring within 24 hours. The
presence of the porphyrin in the tissue may be detected within a few hours
after injection. The presence of the porphyrin in the tissue may persist
for several days to two weeks.
Means for detection of the chemically unaltered porphyrin is primarily by
fluorescence of the compound when it is exposed to ultraviolet light in
the range of 400 to 410 nm. This fluorescence can only be detected by
invasive means such as catherization, surgery, and postmortem examination.
Catheterization is accomplished by use of an ultrathin fiberoptic catheter
(an angioscope), such as the one described in the article "In Vivo
Angioscopy", J. Amer. College of Cardiology 1(5):1311-1314. For
fluoroscopy, an ultraviolet light would be used in place of the visible
light.
Because of the precise imaging by fluorescence of porphyrin containing
plaques, there is a decreased chance of vessel wall perforation at the
time of laser vaporization of the plaques. The difference in fluorescence
between normal tissue and atheromatous plaques is quite striking. A
comparison was made in rabbit aortas and reported in "Florescence of
Experimental Atheromatous Plaques with Hematoporphyrin Derivative" by J.
Richard Spears, Juan Serur, Deborah Shopshire, and Sven Paulin, J. Clin.
Invest. 71:395-399 (February 1983), the teachings of which are
incorporated herein by reference. The only possible disadvantage to this
technique is that the patient may develop a skin photosensitivity
following injection of a hematoporphyrin solution. This appears to be due
to impurities in the hematoporphyrin solution and persists for less than a
week.
In another embodiment of the present invention, the porphyrin is labelled
with a radioactive isotope prior to injection and the relative
concentration of the porphyrin measured using radionuclide scintographic
techniques.
Isotopes suitable for use as labels include those which can be substituted
within the porphyrin ring such as copper, .sup.64 Cu, those molecules
which make up the ring such as carbon and hydrogen, .sup.14 C and .sup.3
H, and those which can be attached by means of side groups, such as
iodine, .sup.125 I. The porphyrin may also be labelled by attaching
radioactively labelled proteins to the porphyrin side groups. The copper
isotope, .sup.64 Cu, has an advantage over radioisotopes such as .sup.14 C
and .sup.3 H in that it has a very short half-life, twelve hours, so the
patient is not exposed to radioactivity for a long period of time.
##STR1##
Hematoporphyrin with metal substituted porphyrin ring.
Iodine, .sup.125 I, has an additional advantage in that it is radio-opaque,
and can be used as a contrast media for X-ray detection.
An example of X-ray detection, Digital Subtraction Angiography (DSA) is
presently used with contrast medias in order to diagnose diseases of blood
vessels of the brain and chest. In this procedure, an X-ray is made of the
area of the body containing the blood vessels prior to injecting the
contrast media. An X-ray taken after injection of contrast media is
compared with the first X-ray and a computer used to construct an image of
the blood vessels. Iodine-labelled porphyrin absorbed by atheromatous
plaques can be used to show the location and quantity of plaques within
these imaged blood vessels by increasing the low level contrast.
In yet another embodiment of the present invention, porphyrins
incorporating isotopes within their structure may be detected using
Positron Emission Tomography (PET). The Positron Emission Tomograph
records the human body's metabolism by measuring traces of "nuclear
annihilations" in body tissue. A patient receives short-lived tracers
which emit particles called "positrons" which collide with electrons less
than a few millimeters away to produce bursts of energy that detectors
record. As these collisions occur repeatedly, the detectors build up a map
of the location of the tracer chemical or the location where it is being
metabolized.
Examples of tracers which may be used with PET include .sup.68 Gallium,
.sup.11 CO, .sup.13 NH.sub.3, and .sup.11 C-glucose.
In a similar embodiment of the present invention, absorbtion of substituted
porphyrins is detected using Nuclear Magnetic Resonance (NMR)
spectroscopy. With the NMR technique, the patient is placed within an
extremely powerful magnetic field and encircling coils pulse the body with
radio waves. Different kinds of atoms in the body re-emit radio signals in
response. A computer processes the signals to form images of the body.
Examples of molecules that can be incorporated within the porphyrin ring
for detection by NMR include maganese, iron, Gadolinium, Chromium, cobalt,
nickel, silver, and europium.
The absorbtion of the porphyrin greatly enhances the signal measured by
NMR. When an excised plaque was measured by NMR, the proton signals
T.sub.1 and T.sub.2 were 450 milliseconds and 86 milliseconds,
respectively. After the plaques were soaked in a porphyrin-maganese
solution, the T.sub.1 and T.sub.2 signals were 70 milliseconds and 2.5
milliseconds, respectively. Some adaptations of the dosage, type of
incorporated metal, etc., may have to be made for the technique to work as
well in an in vivo situation. Unfortunately, once a metal has been
incorporated into the hematoporphyrin, the compound no longer fluoresces
so the NMR and fluoroscopy technique may not be used simultaneously.
In yet another embodiment of the present invention, the absorbtion of
labelled or unaltered porphyrins by atheromatous plaques may be enhanced
by binding to the porphyrin some antibody specific to a component of the
plaques. Monoclonal antibodies would be particularly useful in this
technique due to their extreme specificity. The major component of the
plaques is smooth muscle cells. This is also a component of the normal
blood vessel wall but it is covered by endethelial cells in the normal
vessels. The abnormal endothelium covering the atheromatous plaques
appears to allow porphyrins to penetrate more easily than does the normal
endothelium lining the blood vessels. In the case of smooth muscle cells,
the antibody would probably actually be directed against the myosin. Other
components of the plaques which could serve as antigenic targets include
the elastic elements, collagen, and lipids.
Although this invention has been described with reference to specific
embodiments, it is understood that modifications and variations may occur
to those skilled in the art. It is intended that all such modifications
and variations be included within the scope of the appended claims.
* * * * *
|
|
 |
|
 |
Description |
|
